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An  Experimental   Examination  of  the 

Phenomena  Usually  Attributed  to 

Fluctuation  of  Attention 


The  Intermittence  of  Minimal 
Visual  Sensations 


THESIS 

Presented  to  the  University  Faculty  of  Cornell  University  for  the 
Degree  of  Doctor  of  Philosophy 


BY 
C    E.  FERREE 


Reprinted  from  the  AMERICAN  JOURNAL  OF  PSYCHOLOGY 

January,  1906,  Vol.    XVII,  pp.    81-120 

January,  1908,  Vol.  XIX,  pp.  58-129 


An  Experimental  Examination  of  the 

Phenomena  Usually  Attributed  to 

Fluctuation  of  Attention 


The  Intermittence  of  Minimal 
Visual  Sensations 


THESIS 

Presented  to  the  University  Faculty  of  Cornell  University  for  the 
Degree  of  Doctor  of  Philosophy 


BY 
C.   E.  FERREE 


Of  THE 

UNIVERSITY 

OF 


Reprinted  from  the  AMERICAN  JOURNAL  OF  PSYCHOLOGY 

January,  1906,  Vol.    XVII,  pp.    81-120 

January,  1908,  Vol.  XIX,  pp.  58^29 


QP4U 
F4 


AN  EXPERIMENTAL  EXAMINATION  OF  THE  PHE- 
NOMENA  USUALLY  ATTRIBUTED  TO 
FLUCTUATION  OF  ATTENTION.1 


By  C.  E.  FERREE,  A.  M.,  M.  S. 


TABLE  OF  CONTENTS. 

Page 

Introduction,      ........        82 

I.    Visual  stimuli, 
(i)  Statement  of  theory,  ....  83 


(ii)  Lines  of  proof, 
(iii)  Results:  General,  .... 

(iiii)  General  description  of  method  and  apparatus, 
(v)  Results  in  detail,    ..... 

A.  Involuntary  changes  of  accommodation  are  not  essential, 

B.  A  non-intermittent  stimulus  produces  a  continuous  sensa 

tion  (Electrical), 

C.  All  liminal  stimuli  do  not  fluctuate  (Light),   . 


84 
84 
93 
94 
94 

95 
96 

D.  Adaptation  is  an  intermittent  process  under  the  conditions 

holding  for  fluctuation,    .  .  .  .96 

E.  Adaptation  and  fluctuation  are  identical.     Correspondence 

shown  by : 

(a)  Fading  of  the  stimulus  into  its  proper  gray  during  the 

course  of  a  single  fluctuation,     .  .  .96 

(b)  Comparison  of  fluctuation  time  with  adaptation  time 

using  colors  and  grays,    .  .  .  -97 

(1)  Fluctuation,  .  .  .  .  97 

(2)  Adaptation,    ......        99 

(cj  Combinations  of   stimulus  and   background  that  in- 
fluence adaptation  time  correspondingly  in- 
fluence fluctuation  time,  .  .  .       100 

(1)  Fluctuation,  ......      100 

(2)  Adaptation,    ......       100 

(d)  Method  of  variation  of  areas,        ....       100 

(1)  Fluctuation,  ......      100 

(2)  Adaptation,    .  ...  .  .  .       no 

F.  Adaptation  is  rendered  intermittent  chiefly  by  eye-move- 

ment, ......      in 

(a)  Eye-movement  in  the  horizontal  and  vertical  planes,       114 

(b)  Fluctuation  with  vertical  and  horizontal  arrangement 

of  the  stimulus,     .....       115 

(c)  Adaptation  with  vertical  and  horizontal  arrangement 

of  the  stimulus,     .....       116 

G.  Correspondence  of  fluctuation  with  adaptation  in  indirect 

vision,          ......       116 

(a)  Fluctuation,  ......      116 

(b)  Adaptation,  .......       118 

II.     Cutaneous  stimuli, 
(a) 'Pressure,       .......       119 

(b)  Electro-cutaneous,  ......       119 

the  Psychological  Laboratory  of  Cornell  University. 


207779 


82  FERREE : 

INTRODUCTION. 

It  is  commonly  admitted  by  experimental  psychologists  that 
no  final  explanation  has  as  yet  been  offered  of  those  fluctua- 
tions of  minimal  stimuli  and  minimal  stimulus-differences 
which  go  by  the  general  name  of  'fluctuations  of  attention. '  We 
have  peripheral  theories  and  central  theories  and  mixed,  pe- 
ripheral-central theories;  and,  without  doubt,  we  have  a  good 
deal  of  scattered  knowledge  about  the  conditions  which  under- 
lie the  phenomena  in  certain  of  the  fields  of  sense.  But  this 
chapter  of  psychology  is,  on  the  whole,  still  open.  G.  E. 
Miiller,  for  instance,  writes  in  1904  that  "zu  feststehenden  Re- 
sultaten  von  allgemeiner  Bedeutung  haben  indesseu  diese  Un- 
tersuchungen,  die  sich  in  ihren  Ergebnissen  und  Schlussfolge- 
rungen  vielfach  wiedersprechen,  bisher  noch  nicht  gefiihrt."  l 
Unless  one  is  prejudiced  in  favor  of  some  particular  theory,  one 
cannot  but  subscribe  to  this  opinion. 

Considerations  of  this  sort  led  us  to  begin  a  systematic  investi- 
gation of  the  subject  which  has  extended  from  the  winter  of 
1903  to  the  present  time.  Cutaneous  and  visual  stimuli  were 
used;  but,  since  the  former  gave  uniformly  negative  results,  it 
has  been  possible  to  confine  our  attention  almost  exclusively  to 
the  latter.  We  had  hoped,  likewise,  to  include  auditory  stim- 
uli in  this  series  of  investigations,  but  circumstances  have  ren- 
dered it  necessary  that  we  make  them  the  subject  of  future 
study. 

It  has  become  evident  in  the  course  of  the  work  that  a  com- 
plete account  of  the  fluctuation  of  visual  stimuli  must  take  into 
consideration  also  the  fluctuation  of  the  negative  after-image. 
The  results  of  this  investigation  will  be  made  the  subject  of  a 
second  article,  to  be  followed  by  a  third  in  which  the  conclu- 
sions of  the  two  preceding  articles  will  be  considered  from  the 
standpoint  of  theory  and  in  the  light  of  preceding  work.  The 
present  study  is  a  reproduction,  with  some  changes,  of  a  paper 
read  before  a  meeting  of  experimental  psychologists  held  at 
Cornell  University  in  March,  1904.  It  has  seemed  advisable 
to  publish  it  in  its  present  form,  rather  than  to  wait  for  a  more 
complete  treatment,  as  was  originally  planned,  because  of  the 
evident  revival  of  interest  in  the  problem  and  the  appearance  of 
the  recent  papers  of  Dunlap,2  Killen,8  and  Hammer.4 

For  the  sake  of  clearness,  the  following  order  of  presentation 
will  be  adhered  to  as  closely  as  possible : 

1  Die  Gesichtspunkte  und  die  Tatsachen  der  psychophysischen 
Methodik,  1904,  no. 

2K.  Dunlap:  Psychol.  Rev.,  XI,  308. 

3B.  Killen:  this  Journal,  XV,  512. 

4B.  Hammer:  Zeits.  f.  Psych.,  XXXVII,  363;  cf.  C.  E.  Seashore, 
ibid.,  XXXIX,  668. 


FLUCTUATION  AND  ADAPTATION.  83 

i st.     A  statement  of  theory  sufficiently  comprehensive  to 
render  the  results  intelligible  in  terms  of  it. 
2nd.     A  statement  of  the  lines  of  investigation. 
3d.     A  statement  of  results  in  general. 
4th.     A  statement  of  results  in  detail. 

I.     VISUAL  STIMULI. 

(i)  Statement  of  Theory.  It  is  our  purpose  to  show  in  this 
paper  that  the  intermittences  of  sensation  resulting  from  min- 
imal visual  stimuli  which  have  been  referred  for  explantion  to 
fluctuation  of  attention  are,  in  reality,  simply  adaptation  phe- 
nomena somewhat  obscured  by  the  special  conditions. 

Adaptation  is,  in  itself,  a  continuous  phenomenon,  but  its 
continuity  is  interfered  with  by  eye-movement,1  blinking,  etc. 
Through  these  influences,  probably  essentially  through  that 
of  eye-movement  alone,  it  becomes  an  intermittent  process, 
whether  the  stimulus  be  liminal  or  intensive,  provided  that 
proper  areas  be  used.  The  conditions  are  especially  favorable 
for  short  periods  of  intermittence  when  the  stimuli  are  liminal 
and  of  small  area. 

Eye-movement  tends  to  delay  adaptation  when  the  stimulus 
is  liminal  and  of  small  area.  When  the  stimulus  is  much  above 
the  limen  and  the  area  very  small,  complete  adaptation  is  pre- 
vented, because,  under  these  conditions,  no  one  part  of  the 
retina  is  stimulated  long  enough  to  produce  the  required  physio- 
logical effect.  Also,  under  such  conditions,  it  is  of  very  short 
duration,  when  attained,  because  a  slight  shift  of  the  retina  is 
sufficient  to  produce  a  complete  change  in  the  area  stimulated 
and  thus  to  afford  the  adapted  elements  the  relief  necessary  to 
the  revival  of  sensation.  When,  on  the  contrary,  the  area  is 
very  large,  these  relatively  small  eye-movements  do  very  little 
towards  relieving  the  part  of  the  retina  stimulated;  conse- 
quently, complete  adaptation  takes  place  much  more  quickly, 
and  persists  apparently  indefinitely,  unless  relief  be  similarly 
afforded  by  some  other  agency.  Areas  ranging  from  2  mm.  to 
3-4  cm. ,  viewed  at  a  distance  of  i  meter  or  more,  are  especially 
favorable  for  short  periods  of  intermittence;  hence,  in  the 
previous  investigation  of  this  phenomenon,  it  is  only  natural 
that  they  should  have  been  chosen  and  the  remainder  over- 
looked. 

In  all  experimental  work,  however,  the  conditions  that  are 
unfavorable  to  the  production  of  the  phenomenon  are  as  im- 

1  While  working  with  after-images,  this  past  year,  we  chanced  upon 
another  factor  in  adaptation,  which  (so  far  as  we  can  at  present  tell) 
promises  to  be  important.  Just  how  much  it  bears  upon  the  fluctua- 
tion of  minimal  visual  stimuli  cannot  now  be  stated .  We  hope,  how- 
ever, to  discuss  it  fully  in  the  article  on  the  fluctuation  of  after-images. 


84  FERREE : 

portant  as,  and  often  more  important,  for  theory,  than  those 
more  favorable.  This  proves  to  be  true  in  the  case  of  what 
are  commonly  called  'fluctuations  of  attention. ' 

Our  plan  of  experimentation,  in  general,  has  been  to  isolate, 
and  test  out  separately,  the  probable  factors  involved,  central 
and  peripheral,  endeavoring  so  to  vary  the  conditions  as  to  re- 
lieve introspection  of  any  undue  burden  of  analysis.  Where 
the  possible  factors  are  numerous  and  complex,  introspective 
analysis  unaided  can  scarcely  be  relied  upon  to  solve  the  prob- 
lem. 

(ii)  Lines  of  proof.     It  is  proposed  to  show: 

(i)  That  involuntary  changes  in  accommodation  are  not 
essential  factors  in  the  phenomenon.  (2)  That  a  stimulus 
which  is  not,  in  itself,  intermittent,  acting  upon  the  optic  cen- 
tre, does  not  produce  an  intermittent  sensation.  (3)  That  all 
liminal  stimuli  do  not  fluctuate.  (4)  That  adaptation  is  an 
intermittent  process  under  the  conditions  holding  for  fluctua- 
tion. (5)  That  adaptation  and  fluctuation  are  identical.  (6) 
That  adaptation  is  intermittent  chiefly  because  of  eye-move- 
ment. (7)  That  the  same  correspondence  between  adaptation 
and  fluctuation  obtains  in  indirect  vision. 

(iii)  Results:  General.  We  have  the  following  results  to 
offer  at  this  stage  of  the  work. 

A.  INVOLUNTARY  CHANGES  OF  ACCOMMODATION  ARE  NOT 

ESSENTIAL. 

Aphakial  subjects  experience  these  fluctuations  with  appar- 
ently no  greater  variation  of  phase  than  can  be  accounted  for 
on  the  ground  of  normal  individual  differences.  Hence,  we 
can  conclude  that  involuntary  changes  of  accommodation  play 
no  essential  part  in  the  phenomenon. 

B.  A    NON-INTERMITTENT   STIMULUS    PRODUCES    A    CONTIN- 

UOUS SENSATION. 

A  minimal  and  continuous  light  sensation,  produced  by  elec- 
trical stimulation  of  the  cerebro-retinal  mechanism,  does  not 
fluctuate.  Here  is  a  liminal  stimulus  capable  of  affecting  the 
optic  centre,  and  pouring  in  upon  it,  the  effect  of  which  grad- 
ually dies  out,  but  shows  no  signs  of  intermittence.  This  fact 
would  seem  to  indicate  that  we  must  look  to  the  periphery  for 
an  explanation  of  fluctuation;  for  if  it  were  conditioned  by  cen- 
tral factors,  it  would  be  difficult  to  see  why  an  exception  should 
be  made  in  this  case,  which  is  distinctive  only  in  that  certain 
of  the  peripheral  factors  which  usually  modify  retinal  stimula- 
tion are  omitted. 

C.      NOT  ALL  LIMINAL  STIMULI   FLUCTUATE. 

L/iminal  visual  stimuli  of  large  area,  also  certain  combinations 


FLUCTUATION  AND  ADAPTATION.  85 

of  stimulus  and  background  with  very  small  areas,  do  not 
fluctuate. 

D.    ADAPTATION  is  AN  INTERMITTENT  PROCESS  UNDER  THE 

CONDITIONS  HOLDING  FOR  FLUCTUATION. 

Adaptation,  in  general,  with  areas  equal  to  those  with  which 
fluctuations  are  obtained,  is  a  periodic  phenomenon,  no  matter 
what  the  intensity  of  the  stimulus  used.  The  condition  of  a 
just  perceptible  difference  between  the  stimulus  and  back- 
ground is  favorable,  but  is  by  no  means  essential  to  the  phe- 
nomenon. Any  stimulus  that  will  completely  adapt  into  its 
background  will  do  so  intermittently  within  this  range  of  areas; 
while  a  stimulus  whose  qualitative  relation  to  its  background 
is  such  that  it  will  not  disappear  completely  shows  periodic  in- 
crease and  decrease  in  intensity. 

B.     ADAPTATION  AND  FLUCTUATION  ARE  IDENTICAL. 

Whatever  conditions  relative  to  the  stimulus,  or  to  the  com- 
bination of  stimulus  and  background,  affect  the  adaptation 
time,  produce  a  similar  effect  on  the  fluctuation  time;  the  effect 
showing  itself  either  in  the  phase  of  visibility,  or  in  the  phases 
of  both  visibility  and  invisibility. 

Some  of  the  ways  by  which  this  correspondence  was  shown 
are: 

(a)  Fading  of  the  stimulus  into  its  proper  gray  during  the 
course  of  a  single  fluctuation.  In  the  course  of  a  single  fluctua- 
tion a  colored  stimulus  is  observed  to  fade  into  a  gray,  of  a 
shade  depending  upon  the  color  used,  as  always  happens  in 
complete  color  adaptation.  Further,  the  times  required  for  the 
several  colors  to  fade  sustain  a  very  definite  relation  to  their 
adaptation  times.  In  order  of  their  value  from  least  to  greatest, 
they  are  (for  our  stimuli)  red,  green,  blue  and  yellow.  There 
is  little  difference,  however,  in  the  times  required  for  the  dis- 
appearance of  the  residual  grays  in  each  case.  Moreover,  such 
differences  as  do  occur  are  chance  variations,  as  is  shown  by 
the  following  averages:  red,  1.65  sec.;  green,  1.97  sec.;  blue, 
i. 6 1  sec.;  yellow,  1.65  sec.  Thus  it  would  seem  that  the  dif- 
ference in  the  phases  of  visibility  for  these  four  colors,  which 
is  the  phenomenon  discussed  in  the  next  section,  does  not  de- 
pend upon  their  respective  brightnesses,  but  is  a  duration 
peculiarity  of  the  processes  themselves.1 

1  This  point  is  of  two-fold  importance,  (i)  It  suggests  that  the 
adaptation  time  of  a  color  is  not  a  function  of  its  brightness;  i.  e., 
yellow  and  red  have  in  no  wise  different  adaptation  times  because  of 
their  positions  in  the  white-black  series.  (2)  It  shows  that  the  dif- 
ferent visibilities  in  the  fluctuation  experiments  are  not  conditioned 
by  the  relation  of  the  brightnesses  or  proper  grays  of  the  colors  used 
to  the  background,  but  are  true  expressions  of  characteristic  differences 
in  the  cc4or  processes  themselves. 


86  FERREE : 

(b)  Comparison  of  fluctuation  times  with  adaptation  times  for 
colors  and  grays.     Colors  and  grays  were  found  to  have  an 
order  of  fluctuation  times  corresponding  to  their  adaptation 
times.     Four  colors,  red,  green,  blue  and  yellow,  gave  very 
different  fluctuation  periods  as  compared  with  each  other  and 
with  no.  27  Hering  gray.     The  visibility  times  obtained  were 
in  the  following  order:  red,  green,  blue  and  yellow,  the  yellow 
being  nearly  four  times  as  long  as  the  red.     The  complete 
adaptation  times  for  sheets  of  the  same  colors  were  found  to 
have  the  same  order  of  length  and  a  rough  correspondence  as 
to  ratio  of  length.     Further,  a  striking  fact  came  out  with  re- 
gard to  the  phases  of  invisibility.    Since  red,  for  example,  has  a 
shorter   phase  of  visibility  than   green,  one  might   naturally 
expect  that  its  phase  of  invisibility  would  also  be  shorter  than 
the  invisibility  time  of  green.     The  reverse,  however,  is  true. 
Red  has  a  longer  invisibility  than  green,  and  this  peculiarity  is 
especially  marked  if  one  considers  the  proportionality  between 
the  phases,  i.  e.,  the  ratio  invisibility:  visibility.     The  same 
thing  is  true  of  the  complementaries  blue  and  yellow.    Clearly, 
we  cannot  look  for  a  central  explanation  of  this  peculiarity; 
but  it  seems  just  what  we  might  expect  of  adaptation  from  the 
standpoint  of  the  compensation  theory.     The  recovery  process 
for  the  red  is  the  green  process.     The  green  process  is  longer 
and  seemingly  more  tenacious  than  the  red,  as  is  shown  by  the 
adaptation  experiments  proper,  and  is  further  borne  out  by  the 
longer  duration  of  the  green  after-image.     A  similar  relation 
obtains  in  the  blue-yellow  process.     We  have  now  in  progress 
a  series  of  experiments  that  will  enable  us  to  make  an  exact 
comparison  of  the  recovery  times  for  these  four  colors. 

(c)  Combinations  of  stimulus  and  background  that  influence 
adaptation  times  correspondingly  influence  fluctuation  times.     By 
keeping  the  background  constant  and  varying  the  stimulus,  or 
conversely,  by  keeping  the  stimulus  constant  and  varying  the 
background,  a  difference  in  the  period  of  fluctuation  was  ob- 
tained, showing  itself  chiefly  in  the  phase  of  visibility.     This 
same  thing  held  in  the  recognized   adaptation  experiments. 
The  variations  of  the  phases  of  visibility  and  invisibility  that 
were  produced  in  the  adaptation  experiments  were  produced 
also  in  the  fluctuation  experiments,  the  only  departure  from 
precise  correspondence  being  that  the  differences  were  more 
marked  in  the  former  case,  as  would  be  expected  from  the 
longer  duration  of  the  process. 

(d)  Method  of  areas.     By  adequate  variation  of  the  area 
of  the  stimulus,  the  phase  of  visibility  was  varied  from  quite 
long  with  small  areas  to  nearly  zero  with  large,  while  the  phase 
of  invisibility  ranged  from  very  short  with  small  areas,  to  ap- 
proximate infinity  with  large  areas,  z.  e. ,  the  faded-out  stimu- 


FLUCTUATION  AND  ADAPTATION.        87 

lus  did  not  reappear.  Thus  the  phases  of  visibility  and  invisi- 
bility are,  inversely  to  each  other,  functions  of  the  stimulus 
area.  The  curve  representing  the  phase  of  visibility  starts 
high  on  the  ordinate  and  drops  down  fairly  regularly  to  near 
the  abscissa;  while  the  curve  representing  the  phase  of  invisi- 
bility starts  near  the  abscissa  and  rises  to  infinity.  The  areas 
chosen  do  not  make  the  phase  of  visibility  infinite  with  liminal 
stimuli;  but  it  is  presumable  that  an  area  small  enough  to  do 
this  might  be  found.  The  curve  representing  the  total  period 
begins  high  on  the  ordinate,  bends  down  towards  the  abscissa, 
rises  again,  and  passes  to  infinity.  A  similar  effect,  much 
more  marked,  was  obtained  in  the  adaptation  experiments. 
With  the  smallest  areas  used  above,  the  spot  never  disappeared. 
Thus  the  curve  representing  the  whole  period  starts  at  infinity, 
bends  down,  but  not  so  near  to  the  abscissa  as  before,  rises 
again,  and  passes  back,  but  much  more  irregularly,  to  infinity. 
Further,  if  we  took  areas  sufficiently  large,  not  only  did  the 
faded-out  stimulus  not  become  visible  again  under  the  condi- 
tions of  fixation  observed  in  such  experiments,  but  it  refused 
to  reappear  with  quite  extensive  voluntary  eye-movements. 

Now  there  seems  no  way  of  explaining  these  results  from  any 
peculiarity  of  function  in  the  centre.  In  the  case  of  liminal 
stimuli,  the  intensities  were  chosen  subjectively  equal,  conse- 
quently there  could  be  no  reason  for  a  central  discrimination, 
on  the  ground  of  intensity,  adequate  to  account  for  the  wide 
range  of  variation  obtained;  and  as  for  the  adaptation  experi- 
ments, a  very  flood  of  vaso- motor  waves,1  etc.,  would  scarcely 
suffice  to  wipe  out  stimuli  of  so  great  intensity.  If  it  be  argued 
that  it  is  not  fair  to  attempt  to  carry  over  this  explanation  to 
the  adaptation  experiments,  we  must  reply  that  there  would  re- 
main, then,  the  very  great  difficulty  of  explaining  the  close  cor- 
respondence in  the  results  obtained  in  the  two  series  of  experi- 
ments, if  entirely  different  causes  were  ascribed  to  the  two  sets 
of  phenomena.  In  favor  of  physiological  rhythm,  it  might  be 
said,  however,  that  there  seems  a  bare  possibility  of  establish- 
ing a  connection  between  it  and  eye-movement.  But  only  in 
this  way  could  it  fit  into  a  theory  that  should  explain  all  the 
results  cited  above.  Again,  if  from  any  standpoint  it  be 
argued  that  central  factors2  are  involved  in  eye-movement, 
blinking,  etc.,  and  that  these  influences,  therefore,  make  for  a 
central  theory,  we  reply  that  the  movements  are  more  likely 
to  be  reflex,  made  in  sympathy  with  the  changes  and  needs  of 
the  retina.  But  granted  that  they  are  central,  they  still  play 
no  greater  part  in  the  explanation  of  the  phenomenon  than  is 

TJ.  W.  Slaughter:  this  Journal,  XII,  313. 

2  E.  A.  Pace  :  Philos.  Studien,  VIII,  388;  XX,  232. 


88  FERREE : 

the  case  with  adaptation  in  general.  The  part  played  by  all 
of  these  factors  becomes  a  common  problem,  to  be  investigated 
in  connection  with  adaptation,  and  not  substituted  for  it  as  a 
vera  causa.  There  seems,  likewise,  little  chance  of  explana- 
tion of  these  results  from  the  side  of  attention.  In  fact,  they 
seem  to  be  precisely  contradictory  of  any  theory  that  seeks  its 
account  from  this  source.  Increase  in  area  of  the  stimulus  is 
presumed  to  be  equivalent  to  an  increase  in  intensity,  as  re- 
gards its  noticeability  or  its  efficiency1  for  attention.  Effi- 
ciency, or  whatever  may  be  considered  as  its  equivalent  in 
these  results,  let  the  criterion  be  what  it  will,  is  reduced  to  a 
minimum. 

F.    ADAPTATION  is  RENDERED  INTERMITTENT  CHIEFLY  BY 

EYE-MOVEMENT. 

That  eye-movement2  is  chiefly  responsible  for  the  intermit- 
teuce  of  adaptation  seems  evident  from  the  results.  Blinking 
might  enter  in,  as  an  occasional  factor,  to  delay  adaptation  or 
cause  the  reappearance  of  the  faded-out  stimulus;  but  it  is 
much  too  infrequent  to  explain  all  of  the  reappearances.  Be- 
sides, it  could  offer  no  explanation  for  the  difference  in  the 
times  of  visibility  and  invisibility  for  the  different  areas,  since 

1  There  are,  probably,  two  factors  which  give  an  increased  area  an 
increased  efficiency  for  attention,     (i)  There  is  an  actual  increase  in 
the  intensity  of  the  sensation.      For  example,  when  our  stimulus  was 
obtained  by  light  transmitted  through  opal  glass,  the  source  of  light 
had  to  be  moved  farther  away  in  order  to  give  a  liminal  effect  when 
the  area  of  the  stimulus  diaphragm  was  increased.     I/ikewise,  when 
the  stimulus  was  seen  by  reflected  light,  the  opal  glass  plate  had  to 
be  moved  farther  out  from  the  background  when  the  area  of  the  stim- 
ulus was  increased.     (2)  The  increased  area  occupies  more  of  the  field 
of  vision;  hence  the  rival  area  is  not  only  of  less  extent  (fewer  dis- 
tracting factors,  etc.),  but  is  pushed  more  and  more  into  the  field  of  in- 
direct vision.  The  first  factor  was  ruled  out  by  decreasing  the  intensity 
of  the  stimulus  until  it  was  liminal.  The  second,  however,  operated  to 
give  our  large  areas  greater  efficiency  for  attention.     But  in  spite  of 
this,    the  large  areas,   although   favoring  rapid   adaptation,  gave  us 
minimal  visibility.    Besides  helping  to  make  our  point  for  adaptation, 
this  result  serves  as  a  striking  illustration  of  how  little  the  central 
factors  avail  against  the  peripheral  in  so-called  sensory  attention. 

2  The  other  factor  conditioning  adaptation  is  probably,   likewise, 
essentially  dependent  upon  eye-movement.    At  least,  the  modification 
which  gives  it  a  bearing  upon  this  problem  is  caused  by  eye-move- 
ment.    The  effect  produced  is,  similarly,  a  freshening  of  the  adapted 
elements,  and  will  be  understood,  throughout  the  discussion,  to  sup- 
plement the  change  produced  by  shifting  the  adapted  elements  into  a 
region  of  different  stimulation.     Since  its  action  in  point  of  time  fol- 
lows immediately  upon  eye-movement,  and  does  not  change,  but  only 
supplements,  the  restoration  produced  by  change  of  stimulation,  there 
was  little  apparent  need  of  it  to  explain  the  results  of  the  following 
tables.     Hence,  had  it  not  come  to  light  in  the  work  on  after-images, 
it  probably  would  have  been  entirely  overlooked. 


FLUCTUATION  AND  ADAPTATION.  89 

the  amount  of  relief  afforded  in  each  case  would  be  the  same. 
Eye-movement  alone  seems  adequate  to  do  this. 

That  eye-movement  produces  its  effect  in  the  manner  we 
have  stated  probably  needs  further  proof.  Hess1  has  contended 
that  a  spot  once  adapted-out  will  not  reappear  so  long  as  fixa- 
tion is  held  perfectly  steady;  but  his  experiments  do  not  indi- 
cate how  eye-movement  causes  reappearance.  MacDougall2 
explains  the  effect  of  eye-movement  upon  the  reappearance  of 
minimal  visual  stimuli  on  the  basis  of  innervation.  Innerva- 
tion,  however,  could  not  account  for  phases  of  invisibility 
ranging  from  nearly  zero  to  infinity;  besides  which,  extensive 
voluntary  eye-movements  were  wholly  ineffective  to  revive 
sensation  in  the  case  of  the  largest  areas  used.  Similarly,  the 
mechanical  effects  of  pressure,  etc.,  are  ruled  out.  Hence  we 
seem  not  only  warranted,  but  forced,  to  fall  back  for  explana- 
tion upon  an  actual  shift  of  the  adapted  elements  away  from 
the  area  of  stimulation. 

A  more  direct  experimental  confirmation,  than  was  afforded 
by  the  method  of  variation  of  areas,  of  the  view  that  eye- 
movement  interferes  with  the  course  of  adaptation,  and  is  also 
the  conditioning  factor  for  the  wide  range  of  variability  found 
in  the  phases  of  visibility  and  invisibility  in  the  fluctuation 
experiments,  is  given  by  the  following  results.  An  examina- 
tion of  the  average  frequency  of  eye-movement  in  the  horizontal 
and  vertical  planes  during  fixation  showed  that  three  of  our 
observers  had  a  marked  excess  in  both  frequency  and  range  in 
the  horizontal,  while  the  fourth  had  an  excess  of  frequency  in 
the  vertical,  but  of  range  in  the  horizontal  plane.  This  ap- 
peared to  mean  that,  for  three  observers,  there  was  a  greater 
change  of  stimulation,  and  consequently  greater  relief  for  the 
adapted  elements,  in  the  horizontal  than  in  the  vertical  direc- 
tion, while  the  reverse  was  true,  though  probably  to  a  less 
degree,  for  the  fourth.  To  test  this  interpretation,  stimuli 
longer  than  broad  were  used,8-?,  g.,  slips  of  paper  5  mm.  x 
40  mm.  When  these  were  placed  with  the  longer  dimension 
vertical,  the  shorter  dimension  would  fall  in  the  direction  of 
greater  unsteadiness  of  fixation  for  the  three  observers  who 
had  the  excess  of  eye-movement  in  the  horizontal  plane.  Con- 
sequently, a  maximal  interference  with  adaptation  for  these 
stimuli  would  be  obtained,  and  one  might  expect  an  increase 
in  the  phase  of  visibility  and  a  decrease  in  the  phase  of  invisi- 
bility. On  the  other  hand,  if  the  longer  dimension  were  placed 
in  the  horizontal  and  the  shorter  in  the  vertical  plane,  a  mini- 

1  C.  Hess:  von  Graefe's  Archiv,  Xly,  2,  274. 

2  W.  MacDougall:  Mind,  XI,  316;  XII,  289. 

8This  procedure  was  suggested  by  Professor  Iy.  Witmer,  of  the  Uni- 
versity of  Pennsylvania. 


QO  FERREE : 

mal  interference  possible  to  these  stimuli  would  be  secured,  and 
a  decrease  in  the  phase  of  visibility  and  an  increase  in  that  of 
invisibility  should  ensue.  For  the  fourth  observer,  with  the 
stimulus  arranged  as  described  above,  the  reverse  should  be 
true;  but  probably  not  in  so  marked  a  degree,  since  his  range 
was  greater  in  the  horizontal,  and  this  fact  to  a  certain  extent 
counteracted  the  effect  of  frequency.  This  observer  also  had 
an  astigmatism  in  the  vertical  plane,  which  caused  the  stimulus 
to  become  spreading  and  diffuse  in  the  horizontal,  a  result 
equivalent  to  greater  breadth  for  adaptation. 

That  these  methods  of  arrangement  of  stimulus  caused  a 
marked  change  in  the  phases  of  visibility  and  invisibility  for 
each  observer  will  be  seen  by  inspection  of  the  Tables.  Indeed,  the 

visibility  -v-invisibility 
correspondence  between  the  quantities:  visibility.H- in  visibility' 

and  7 — ^          ,   is  much  closer  than  was  anticipated, 
frequency 1 

G.     CORRESPONDENCE  OF  ADAPTATION  WITH  FLUCTUATION 

IN  INDIRECT  VISION. 

To  show  that  fluctuation  in  indirect  vision  is  not  a  special 
phenomenon,  but  that  the  correspondence  between  adaptation 
and  fluctuation  obtains  here  as  well  as  in  direct  vision,  the  fol- 
lowing set  of  experiments  was  carried  out.  (i).  Beginning 
with  direct  vision,  a  liminal  stimulus  was  moved  successively 
4,  8,  12,  16,  etc.,  cm.  towards  the  periphery,  and  records  were 
obtained  at  each  point.  A  parallel  set  of  records  was  obtained 
with  the  same  stimulus  at  full  intensity.  Both  sets  of  records 
showed  a  fairly  regular  decrease  of  visibility  and  increase  of 
invisibility  as  the  stimulus  was  moved  towards  the  periphery. 
The  adaptation  times  obtained  in  a  separate  series  of  experi- 
ments with  the  same  stimulus  also  showed  a  corresponding  de- 
crease from  direct  vision  to  periphery. 

(2).  An  increase  of  area  with  liminal  stimuli  in  indirect 
vision  gave  a  decrease  of  visibility  and  an  increase  of  invisibil- 
ity, very  much  the  same  as  was  obtained  for  direct  vision. 

There  seems  little  doubt  that  all  the  results  secured  for  direct 
vision  could  have  been  paralleled  for  indirect  vision.  The  above 
series,  however,  satisfied  us  that  the  phenomenon  here  is  essen- 
tially the  same.  It  seems,  then,  that  the  conclusion  is  justified 
that  adaptation  causes  the  disappearance  of  the  stimulus,  and 
unsteadiness  of  fixation  the  wide  range  of  visibility  and  invisi- 
bility in  case  of  different  areas,  and  the  restoration  when  com- 
plete adaptation  has  set  in;  and  that  this  effect  is  due  to  relief 
of  adapted  elements  by  actual  shift  away  from  the  area  of  stim- 
ulation, or  rather  into  a  region  of  different  stimulation.1 

1  Together  with  the  supplementary  factor  mentioned  but  not  speci- 
fied above, — if  this  prove  to  have  the  efficacy  which  we  now  incline 
to  ascribe  to  it. 


FLUCTUATION   AND   ADAPTATION.  9! 

H.      FACTS   OF  MINOR   IMPORTANCE. 

The  following  facts  of  minor  importance  may  also  be 
cited. 

(a)  Result  of  increasing  the  distance  of  the  observer.     The 
effect  of  increasing  the  distance  of  the  observer  from  the  stim- 
ulus was  tried     The  area  subtended  by  the  stimulus  on  the 
retina  follows  the  law  of  inverse  squares.     Although  the  phase 
of  visibility  increased  and  the  phase  of  invisibility  decreased 
with  the  increase  of  the  observer's  distance,  still  the  results  did 
not  at  all  closely  follow  those  obtained  by  the  corresponding 
variations  of  area  observed  at  a  distance  of  i  meter.  The  phase 
of  invisibility  increased  much  more  rapidly  with  the  increase 
of  distance  than  was  demanded  by  the  law  of  inverse  squares. 
This  seems  to  argue  in  favor  of  eye-movement;  for  the  greater 
the  observing  distance,  the  greater  is  the  shift  of  the  adapted 
elements  away  from  the  stimulating  area  with  each  eye-move- 
ment; hence  the  greater  is  the  interference  with  the  course  of 
adaptation. 

(b)  Connection    between    reappearance,    and  conscious  eye- 
movement  and  blinking.     Experiments  for  recording *  the  con- 
nection  between  reappearance   and   conscious  eye-movement 
and  blinking  showed  coincidence  in  from  one-third  to  one-half 
the  total  number  of  cases. 

(c)  Effect  of  moving  the  eyes  voluntarily.     Records  of  series 
in  which  an  observer  purposely  moved  his  eyes  at  short  inter- 
vals showed   very   few  fluctuations.     Another  observer  was 
directed  to  relieve  the  strain  when,  and  as,  impulse  directed. 
No   fluctuations  were  experienced   in  one  revolution  of  the 
drum:   102  sees. 

(d)  Effect  of  momentary  cessation  of  the  stimulus.     Anything 
else  that  temporarily  relieved  the  retina,  such  as  the  interposi- 
tion of  some  object  between  the  source  of  light  and  the  screen 
when  the  spot  was  made  visible  by  transmitted  light,  caused 
reappearance  when  the  spot  had  vanished,  and  delayed  disap- 
pearance when  the  spot  was  visible. 

(e)  Influence  of  practice.    An  inexperienced  observer  usually 
obtained  longer  times  of  visibility  and  shorter  times  of  invisi- 
bility until  a  certain  stage  of  practice  was  reached.     Some,  in- 
deed, were  unable  at  first  to  get  fluctuations  at  all.     This  is 
precisely  what  would  be  expected  as  the  result  of  unpractised 
fixation  upon  adaptation.     Further,  the  result  seems  incom- 
patible with  the  theory  of  fluctuation   of  attention,  for  one 

1  The  method  of  recording  was  simple.  When  reappearance  came 
•with  conscious  eye-movement  or  blinking,  O  substituted  for  the  usual 
release  of  the  key  an  extra  pressure  and  immediate  release.  With 
practice,  this  method  offered  little  if  any  distraction. 


92  FERREE : 

would  expect  practice  to  increase,  certainly  not  to  diminish, 
efficiency  of  attention. 

Again:  towards  the  close  of  a  sitting,  with  one  of  our  ob- 
servers, the  phase  of  visibility  began  to  lengthen  and  the  phase 
of  invisibility  to  decrease  very  perceptibly:  in  a  few  cases  so 
much  so,  that  disappearance  did  not  come  at  all.  At  these 
times  the  observer  complained  of  eye-fatigue  and  inability  to 
fixate  steadily.  This  result,  too,  testifies  for  adaptation  and 
against  central  factors. 

(f)  Introspective  evidence.  Introspection  also  furnishes  val- 
uable evidence.  For  all  observers  the  spot  faded  gradually,1 
and  as  this  process  went  on  the  strain  of  attention  increased, 
reaching  its  maximum  with  the  disappearance  of  the  stimulus, 
and  continuing  until  reappearance,  when  momentary  relief  was 
experienced.  The  natural  attitude  of  our  observers  seems  to 
have  been  to  hold  the  sensation  as  long  as  possible.  Hence  it 
was  to  be  expected  that  the  strain  should  increase  with  the  de- 
crease of  the  sensation.  Had  their  attitude  been  different,  had 
they,  for  instance,  been  instructed  that  disappearance  was  the 
thing  to  be  expected  and  attained,  it  is  possible  that  relief 
might  have  come  with  invisibility :  that  relaxation  of  attention 
might  then  have  ensued.  Even  so,  it  would  have  been  the  re- 
sult and  not  the  cause  of  the  disappearance.  Moreover,  the 
conditions  of  the  experiment  make  for  stimulation  rather  than 
for  fatigue  of  attention.  The  constantly  changing  stimulus, 
the  unexpected  reappearances,  etc.,  are  attention-compelling  to 
a  high  degree.  There  is  no  monotony.  There  are  rather  ele- 

1  The  conflicting  reports  on  this  point  in  the  literature  have  prob- 
ably been  due  to  the  peculiar  difficulties  attending  observation  with 
the  Masson  disk.  With  stationary  stimulus  and  background,  such  as 
were  used  by  us,  there  is  no  doubt  that  the  stimulus  disappears  grad- 
ually. If,  on  occasion,  the  actual  change  in  intensity  could  not  be 
detected,  the  disappearance  was  gradual  and  progressive  in  point  of 
area,  the  background  encroaching  upon  the  stimulus  from  one  direc- 
tion or  another.  More  will  be  said  about  this  type  of  disappearance 
in  a  later  article. 

If  it  be  contended  that  stimuli  whose  intensity  can  be  detected  in 
decrease  are  not  liminal,  we  reply  that,  in  practice,  just  noticeability 
is  not  so  consistently  obtained  that  the  detection  is  impossible.  We 
have  worked  most  carefully  to  get  this  degree  of  intensity,  approach- 
ing the  point  from  either  direction,  and  still  the  observer  would  report, 
during  the  course  of  the  fluctuation,  that  the  stimulus  faded  out. 
What  holds  of  our  stimuli  has  probably  held  also  of  others;  for  we  un- 
doubtedly succeeded  in  getting  finer  adjustments  of  intensity  with  the 
arrangement  finally  adopted  than  was  possible  with  the  Masson  disk. 
With  the  Masson  disk  itself  fading  was  recorded  by  our  observers. 

Dealing,  as  we  did,  with  many  degrees  of  intensity,  facility  for 
judging  intensity  changes  was  naturally  acquired.  During  this  time, 
besides,  two  of  our  observers  were  regularly  working  on  the  deter- 
mination of  visual  limens. 


FLUCTUATION   AND   ADAPTATION.  93 

ments  of  fascination.  As  one  observer  stated,  '  'one  is  always 
on  the  alert  to  see  what  will  happen  next."  In  fact,  were  we 
endeavoring  to  demonstrate  an  unwavering  attention,  scarcely 
a  better  set  of  conditions  could  have  been  selected. 

It  will  be  understood  here,  as  elsewhere  in  the  discussion, 
that  our  contention  is  not  that  attention  does  not  fluctuate.1 
That  is  a  question  aside.  We  are  merely  concerned  with  show- 
ing that  certain  phenomena,  that  have  usually  been  attributed 
to  fluctuation  of  attention  and  cited  as  its  classical  demonstra- 
tion, are  to  be  otherwise  explained.  That  there  is  fluctuation 
on  the  content  side  of  consciousness  goes  without  saying:  the 
sensation  comes  and  goes.  But  we  believe  not  only  that  this 
fluctuation  is  to  be  explained  wholly  by  reference  to  the  sense 
process,  but  also  that  the  associative  factors  that  aid  in  the  ex- 
altation of  the  sensation  are  all  the  more  active  because  of  this 
sinking  of  the  content  below  the  limen  on  the  peripheral  side. 
We  may  add  then  that,  in  so  far  as  the  facilitation  of  the  process 
elevated  to  prominence,  or  the  inhibition  of  other  processes, 
depends  upon  associative  factors,  it  should  be  maintained  that 
the  conditions  of  these  experiments  make  for  an  exalted  and 
sustained  attention. 

(iiii).  General  description  of  method  and  apparatus.  Before 
going  more  into  detail  as  to  method  and  results,  we  may  re- 
mark that  all  devices  that  did  not  produce  decided  changes  of 
result  have  been  considered  as  worthless  for  yielding  evidence 
in  a  case  where  without  change  in  the  experimental  conditions 
the  variations  are  so  considerable.  One  finds  cited  in  the  lit- 
erature, as  due  to  some  change  in  method  or  in  support  of  some 
particular  theory,  variations  no  greater  than  our  records 
showed  from  day  to  day  without  any  change  in  the  experi- 
mental conditions.  There  is,  in  dealing  with  this  problem, 
especial  need  for  clearly  cut  and  decisive  methods  of  experi- 
mentation, as  well  as  for  extreme  caution  in  referring  slight 
changes  in  result  to  a  variation  of  experimental  conditions. 

AIn  so  far  as  attention  is  considered  as  a  state  or  mode  of  conscious- 
ness, it  may  be  said  to  fluctuate.  But  interpreted  in  this  sense,  it  is 
ruled  out  for  purposes  of  explanation.  We  must  look,  instead,  to  the 
processes  concerned  in  giving  this  particular  state  or  mode  to  con- 
sciousness. The  above  paragraph  is  written  from  the  point  of  view  of 
the  central  processes  involved.  If  we  are  to  investigate  the  action  of 
these  processes,  it  would  be  well  to  have  consciousness  as  purely  cen- 
tral as  possible,  i.  e.,  ideas  should  be  worked  with,  instead  of  sense 
perceptions.  The  changing  content  given  by  the  peripheral  process  is 
fatal  to  the  determination  whether  the  central  processes  will  act  con- 
tinuously for  any  length  of  time  in  a  given  relation. 

In  general,  we  probably  recognize  too  little  the  difference  between 
attention  where  the  content  is  peripheral,  and  attention  where  it  is 
central.  The  distinction  should  undoubtedly  be  made  in  any  discus- 
sion of  fluctuation. 


94  FERREE : 

We  have  even  found  it  necessary  not  to  be  obliged  to  compare 
results  obtained  at  different  sittings,  because  of  the  subjective 
changes  that  occurred  from  time  to  time  in  spite  of  experi- 
mental control.  Our  comparisons  have,  therefore,  been 
planned  in  series  to  be  finished  at  a  single  sitting,  and  the 
order  of  their  presentation  has  been  changed  so  as  to  compen- 
sate as  much  as  possible  for  probable  changes  in  the  condition 
of  the  eyes  and  fixation-apparatus  from  the  beginning  to  the 
close  of  the  period.  Series,  then,  were  compared  from  day  to 
day,  rather  than  the  members  needed  to  make  a  single  series. 
For  registration,  throughout  all  of  the  work,  a  Ludwig- 
Baltzar  kymograph  was  used;  together  with  a  Marey  tambour 
and  bulb,  whereby  the  entire  course  of  the  fluctuation  as  well 
as  mere  appearance  and  disappearance  could  be  traced,  when 
desired;  and  an  electromagnetic  time-marker  in  circuit  with  a 
metronome,  enclosed  in  a  soundless  box.  All  of  this  apparatus 
was  screened  from  the  observer  by  a  sliding  curtain.  The 
work  was  done  mostly  in  a  long  room,  the  'reaction  room,' 
with  the  windows  all  at  one  end.  Thus  cross-lights,  unequal 
illumination  of  the  background,  etc.,  could  be  avoided.  The 
observer  sat  with  his  back  to  a  high  window  and  his  head  in  a 
head- rest  fastened  to  the  edge  of  a  long  table,  along  which 
the  frame  bearing  the  stimulation  apparatus  was  moved  as 
required.  The  time  unit  throughout  is  i  sec. 

(v)  Results.  (In  detail.'}  It  is  scarcely  necessary  to  men- 
tion that  the  results,  unless  otherwise  stated  in  the  tables,  are 
averages  obtained  from  a  large  number  of  records.  In  the 
main,  throughout  the  work,  they  were  confirmed  not  only  by 
the  writer  and  the  observers  cited:  viz..  Misses  Fitch  (F)  and 
George  (Ge)  and. Messrs.  Sabine  (S)  and  Galloway  (Ga), 
but  also  by  a  number  of  the  students  of  the  junior  training 
course,  either  as  a  part  of  their  regular  work,  or  as  substituted 
for  it.  Where  results  have  not  been  obtained  from  all  of  the 
regular  observers,  this  has  been  due  solely  to  lack  of  time.  S 
and  Ge  gave  the  least  and  Ga  the  longest  time  to  the  work. 
All  four  observers  were  students  in  the  department  of  psychol- 
ogy, and  had  had  laboratory  training.  Ga  had  also  had  ex- 
perience with  the  problem,  both  as  experimenter  and  observer, 
at  the  University  of  Michigan. 

A.     INVOLUNTARY  CHANGES  OF  ACCOMMODATION  ARE  NOT 

ESSENTIAL. 

Two  aphakial  subjects  were  experimented  upon.  One  of 
them  had  so  little  accommodation  that  words  in  fine  print  could 
not  be  moved  more  than  2  mm.  farther  from  or  nearer  to  his 
point  of  clearest  vision  (determined  by  the  focus  of  his  glasses) 
without  becoming  less  distinct.  His  head  was  clamped  in  a 


FLUCTUATION  AND  ADAPTATION.        95 

head-rest,  and  the  card  slid  along  a  meter  rod  at  the  level  of 
his  eyes  in  the  median  plane.  Every  precaution  was  taken  to 
secure  accuracy.  It  may  safely  be  said  that  the  man  was  prac- 
tically without  accommodation.  As  was  stated  before,  the 
results  obtained  from  both  of  these  men  were  uniformly  nega- 
tive, i.  £.,  no  greater  variations  were  found  than  can  be  ex- 
plained on  the  ground  of  normal  individual  differences. 

B.      A   NON-INTERMITTENT  STIMULUS   PRODUCES   A   CONTINU- 
OUS  SENSATION. 

The  well-known  fact  that  make  or  break  of  a  direct  current 
produces  a  flash  of  light,  if  the  electrodes  are  properly  applied, 
led  us  to  believe  that,  if  the  current  were  rapidly  interrupted, 
these  flashes  might  be  caused  to  fuse  into  a  continuous  sensa- 
tion. This  proved  to  be  true.  An  interrupter  so  constructed 
that  six  makes  and  breaks  occurred  with  every  revolution  of 
the  interrupting  cylinder  was  used.  It  was  driven  by  a  motor, 
and  its  speed  of  revolution  was  regulated  by  a  transformer,1 
so  finely  graduated  that  a  change  of  a  single  interruption  could 
be  obtained.  In  circuit  with  the  observer  and  the  battery  was 
inserted  a  resistance  rack  of  German  silver  wire,  also  a  West- 
inghouse  ammeter  graduated  in  milliamperes.  By  this  ar- 
rangement it  was  possible  to  keep  the  current  flowing  through 
the  circuit  absolutely  constant.  A  speed  indicator  was  also 
used.  This  was  rendered  necessary  for  the  double  reason  that 
the  quality  of  the  stimulus  depended  upon  the  rate  of  inter- 
ruption, and  that  any  change  in  the  rate  influenced  the  amount 
of  current  flowing  through  the  circuit.  This  latter  phenome- 
non was  probably  due  to  induction  effects  in  the  coils  of  wire 
used.  The  number  of  Icelandic"  cells  required  to  produce  the 
stimulation  was  usually  eight,  although  as  few  as  four  and  as 
many  as  twelve  were  used  for  different  observers.  The  current 
flowing  through  the  circuit,  when  liminal  effects  were  obtained, 
ranged  from  one  to  two  milliamperes.  The  one  electrode  was 
placed  in  the  hand  and  the  other,  a  sponge  electrode,  above 
the  e>re  on  the  nasal  side.  The  observer  was  stationed  in  the 

1  The  speed-transformer,  made  to  our  order,  was  in  the  form  of  a 
segment  of  a  cone,  with  a  grooved  surface  for  the  retention  of  the 
motor  and  interrupter  belts.  The  dimensions  of  the  segment  were 
such  that  the  decrease  in  circumference  from  groove  to  groove  was 
very  small.  This  arrangement,  together  with  graduated  pulleys  on 
the  motor  and  interrupter,  made  very  slight  changes  of  speed  possible. 

The  interrupter  consisted  of  two  brass  cylinders  with  six  equal  open 
and  closed  spaces  on  either  surface.  An  insulating  cross-section  sep- 
arated the  two  cylinders.  This  duplicate  arrangement  was  not  neces- 
sary, except  that  it  made  the  connections  more  convenient  for  our 
purpose,  and  that,  by  a  proper  setting  of  the  brushes,  the  instrument 
could  also  be  used  as  an  alternator.  The  motor  and  interrnpter  were 
mounted  on  sliding  frames,  in  order  that  the  belts  might  be  kept  taut. 


96  FERREE : 

dark-room,  and  allowed  to  adapt.  Then  the  current  was  ap- 
plied, and  carefully  worked  down  until  liminal  effects  were 
produced.  The  sensation  chosen  for  observation  was  of  the 
nature  of  an  irregular  patch  or  cloud  of  light,  varying  in  color 
for  different  observers  through  violet,  blue,  and  yellow. 

The  sensation,  when  liminal,  usually  lasted  about  30  seconds, 
gradually  fading  out,  and  in  no  case  reappearing  however  long 
the  current  was  applied.  The  effects  obtained  at  different  rates 
of  interruption  show  differences.  Lower  rates  usually  pro- 
duced a  series  of  flashes,  in  which  more  or  less  irregular  pat- 
terns were  made  out.  A  little  higher  rate  produced  bars  on  a 
colored  background.  With  a  still  higher  rate,  the  bars  as- 
sumed a  radial  position  around  a  dark  opening  fringed  with 
colored  light.  Here  began  the  transition  stage.  An  increase 
now  gradually  changed  the  effect  to  an  uniformly  colored  field. 
This  fusion  usually  came  at  rates  ranging  from  85-100  inter- 
ruptions per  second.  One  observer  at  80  saw  a  dark  violet 
field;  at  85,  purple;  at  100,  blue;  and  from  120-162,  yellow. 
It  would  be  interesting  to  discover  whether  there  is  a  definite 
order  in  the  succession  of  colors  for  all  observers  as  the  rate  is 
increased.  The  point  being  merely  incidental  to  our  purpose, 
the  investigation  was  not  carried  far  enough  to  determine  this. 

That  the  retina  is  stimulated  is  indicated  by  the  following 
experiment.  A  rate  of  interruption  was  chosen  that  would 
produce  bars.  The  observer  stimulated  each  eye  separately 
and  noted  the  patterns  obtained.  Then  the  electrodes  were 
applied  above  both  eyes  simultaneously.  It  so  happened  that 
the  bars  for  one  eye  were  inclined  towards  the  horizontal,  and 
for  the  other  towards  the  vertical.  When  both  eyes  were 
stimulated  at  once,  and  the  fields  superposed,  the  two  patterns 
still  remained  distinct,  with  the  bars  set  obliquely  to  each 
other.  As  to  whether  the  visual  substance  was  involved,  the 
experiment  showed  that  there  was  always  an  after-effect,  which 
behaved  much  as  after-images  do.  However,  there  was  rarely 
any  trace  of  complementary  coloring.  In  any  event,  the  result 
goes  to  prove  that  a  continuous  stimulation,  reaching  the  optic 
centre,  does  not  produce  an  intermittent  sensation. 

The  data  of  C.  and  D.  are,  for  convenience,  subsumed  under 
E.,  to  aid  in  showing  the  correspondence  between  adaptation 
and  fluctuation. 

C,  D,  E.    ADAPTATION  AND  FLUCTUATION  ARE  IDENTIC AI,. 
CORRESPONDENCE  is  SHOWN  BY  : 

(a)  Fading  of  the  stimulus  into  its  proper  gray  during  the 
course  of  a  single  fluctuation.  A  Masson  disk  of  the  standard 
dimensions  was  used.  The  colors  (Hering  standard)  were 


FLUCTUATION  AND  ADAPTATION. 


97 


red,  green,  blue,  and  yellow.  The  background  was  neutral 
engine-gray,  darkened  by  180°  of  velvet  black. 

The  change  into  a  gray  differing  from  the  background  was 
first  reported  by  Dr.  Bentley.  Better  to  bring  out  the  phenom- 
enon, a  comparison  ring  of  gray  was  made  concentric  with  the 
colored  ring.  The  judgment  was  difficult,  and  it  has  not  been 
possible  as  yet  to  repeat  the  experiment  under  more  favorable 
conditions.  The  grays  into  which  the  colors  changed  were 
judged  of  different  brightnesses  in  the  order,  from  least  to 
greatest,  of  blue,  red,  green,  and  yellow.  These  grays  corres- 
pond to  those  obtained  when  these  particular  colors,  saturated, 
were  adapted  down. 

Adaptation,  then,  evidently  carries  the  colors  to  the  limen. 
That  it  is  also  adequate  to  get  rid  of  the  gray  remaining  can- 
not be  questioned.  Consequently,  it  does  not  seem  necessary 
to  supply  another  process  to  complete  the  disappearance,  espe- 
cially when  there  is  nothing  in  the  course  of  the  phenomenon 
to  indicate  the  need  of  such  a  supplement.  Introspection 
shows  the  change  from  start  to  finish  to  be  uniform  and  contin- 

TABI,B  I.1 

Ga.    Fading  of  a  color  into  Us  proper  gray  during  the  course  of  a  sin- 
gle fluctuation. 


Stimulus 

Number  of 
Fluctua- 
tions 

Changes 
to  Gray 

Vis. 
Color  Gray 

Invis. 

Order  of 
Brightness 
of  Gray 

Red,  2x5  mm. 
Green,  "     " 
Blue,     "     " 
Yell'w,"     " 

21 

U 
13 

14 

8 

7 

I 

2.4 
2.99 
3-57 
3-70 

1.65 
1.97 
1.61 
1.65 

1.99 
1.79 
2.04 

2.02 

3d- 

2d. 
4th. 

ISt. 

uous.  It  will  be  noticed  that  the  difference  between  the  total 
phase  of  visibility  for  the  four  colors  in  Table  I  is  not  nearly  so 
great  as  it  is  in  Tables  II,  III,  IV,  and  V.  The  recovery-pecu- 
liarities characteristic  of  adaptation  are  also  much  less  notice- 
able in  the  phases  of  invisibility.  This  difference  in  result  is 
always  found  when  the  data  for  the  Masson  disk  and  the  sta- 
tionary system  are  compared. 

(b)  Comparison  of  adaptation  time  and  fluctuation  time  for 
colors  and  grays.  (i)  Fluctuation.  Squares  of  paper  were 

1The  writer  must  apologize  to  the  reader  for  the  ragged  appearance 
of  this  and  the  following  Tables,  owing  to  the  various  number  of  deci- 
mal places  to  which  the  calculations  have  been  carried  out;  and  must 
also  deprecate  any  claim  to  especial  accuracy  in  the  case  of  the  longer 
decimals.  He  had  intended  to  round-off  the  figures  to  two  places,  but 
this  was  inadvertently  omitted.  Rather  than  delay  the  printers,  he 
has  allowed  the  Tables  to  stand  as  they  were  in  MS. 


98 


FERREE 


pasted  upon  gray  card-board  and  placed  behind  an  opal  glass 
plate.  They  were  thus  seen  by  reflected  light  through  the  opal 
glass.  The  intensity  was  easily  regulated  by  slight  changes  in 
the  distance  of  the  plate  from  the  card-board.  Different  makes 
of  standard  colors  were  used  at  different  times.  The  stimuli 
for  the  following  Tables  were  cut  from  Milton-Bradley  papers. 
The  size  of  the  squares  was,  in  each  case,  2  cm.  x  2  cm.,  and 
the  distance  of  the  observer  i  meter.  All  other  conditions  were 
the  same  throughout. 

TABI,E  II. 

Ga.     Comparison  of  fluctuation  time  with  adaptation  time  using  colors 

and  grays.    Fluctuation:  showing  that  visibility  and  invisibility 

have  characteristic  adaptation  and  recovery  peculiarities. 


Stimulus 

Vis. 

M.  V. 

Invis. 

M.  V. 

Vis.: 
Invis. 

Invis.: 
Vis. 

Period 

Gray,     2x2  cm. 

4-34 

1.045 

3-50 

.727 

1.24 

7.84 

Red,         "      " 

2.26 

•53 

3-48 

.108 

.649 

1-539 

5-74 

Green,     "      " 

3-7i 

•954 

3-058 

.80 

1.213 

.824 

6.768 

Blue,        "      " 

5-68 

1.244 

3-755 

1.181 

I-5I3 

.661 

9-435 

Yellow,  "      " 

8-375 

2.075 

3-46 

•337 

2.42 

•413 

"•835 

TABI,B   III. 

S.     Comparison  of  fluctuation  time  with  adaptation  lime  using  colors 

and  grays.    Fluctuation:  showing  that  visibility  and  invisibility 

have  characteristic  adaptation  and  recovery  peculiarities. 


Stimulus 

Vis. 

M.  V. 

Invis. 

M.  V. 

Vis.: 
Invis. 

Invis.: 

Vis. 

Period 

Gray,     2x2  cm. 

3-47 

1-053 

2.31 

•533 

1.502 

5-78 

Red,         "      " 

•338 

2.8 

•527 

.4761 

2.025 

4-133 

Green,     "      " 

2.806 

-506 

2.431 

•575 

I-I54 

.866 

5-237 

Blue,        "      " 
Yellow,  "      " 

3-2 

-836 
1-257 

1.779 
1.238 

•326 
•537 

1.799 
4-504 

-562 

.221 

4-979 
6.814 

TABI.B  IV. 

Ge.     Comparison  of  fluctuation  time  with  adaptation  time  using  colors 

and  grays.     Fluctuation:  showing  that  visibility  and  invisibility 

have  characteristic  adaptation  and  recovery  peculiarities . 


Stimulus 

Vis. 

M.  V. 

Invis. 

M.  V. 

Vis.: 
Invis. 

Invis.: 
Vis. 

Period 

Gray,      2x2  cm. 
Red,         "      " 

3-715 
1.566 

:$ 

2.028 
5-95 

.917 
i.  608 

1.832 
.2632 

3-799 

5-743 
7-5i6 

Green,     "      " 

3-17 

.9b 

5-6 

2.41 

.566 

1.767 

8-77 

Blue,        "      " 

4.471 

1-157 

5-042 

1.142 

.886 

1.127 

9-5I3 

Yellow,  "      " 

7.2 

1-563 

1-354 

•355 

.288 

.188 

8-554 

FLUCTUATION  AND   ADAPTATION. 
V. 


99 


F.     Comparison  of  fluctuation  time  with  adaptation  time  using  colors 

and  grays.    Fluctuation:  showing  that  visibility  and  invisibility 

have  characteristic  adaptation  and  recovery  peculiarities. 


Stimulus 

Vis. 

M.  V. 

Invis. 

M.  V. 

Vis.: 
Invis. 

Invis.; 
Vis. 

Period 

Gray,     2x2  cm. 
Red,         "      " 
Green,     "      " 
Blue,        "      " 
Yellow,  "      " 

3.8066 
2.4812 

3-55 
3-586 
4.716 

.786 
.287 
.462 

•653 
.791 

2-54 
3-362 
2.85 

2-793 
2.141 

.466 
•  568 
•443 

'366 

1.498 
•737 
1.245 
1.283 

2.202 

1-355 
.802 

•779 
•453 

6.3466 

5-843 
6.40 

6-379 
6.857 

Attention  is  called  again  to  the  fact  that,  as  would  be  ex- 
pected from  the  compensation  theory,  red  and  blue  have  longer 
phases  of  invisibility  and  shorter  phases  of  visibility,  respect- 
ively, than  green  and  yellow.  The  relative  value  of  the  invis- 
ibilities as  compared  with  the  visibilities  in  each  case  is 
expressed  by  the  ratio  invisibility  :  visibility. 

(2)     Adaptation.     To  test  the  correspondence  of  these  re- 
sults with  those  obtained  from  adaptation,  sheets  of  colors  of 
the  same  make  were  placed  behind  lightly  frosted  glass  and 
observed   at  distances  ranging   from   2-3   meters.     Just  how 
much  the  intensity  was  lowered  by  these  conditions  we  are  not 
able  to  say, — probably  not  one  half.     This  does  not  matter, 
however,  so  long  as  each  color  was  tested  under  precisely  the 
same  conditions,  since  only  comparative  values  were  wanted. 
The  following  results  were  obtained  : 
Obs.  G.     Distance  :   235  cm.     Time  unit :  i  sec. 
Red  41 

Green  55 

Blue  78 

Yellow  263 

Because  of  the  severe  eye-strains,  the  intensity  was  further 
reduced  for  F  by  placing  the  color  1 1  cm.  behind  the  frosted 
glass. 

Obs.  F.     Distance  :  235  cm.     Time  unit :  i  sec. 
Red  25 

Green  41 

Blue  58 

Yellow  225 

For  5,  the  color  was  placed  19  cm.  behind  the  frosted  glass. 


Obs.  S.     Distance :  300  cm. 
Red 
Green 
Blue 
Yellow 


Time  unit 

19 

52 
1 60 
196 


i  sec. 


100  FERREE : 

The  order  is  the  same  as  was  obtained  in  the  fluctuation  ex- 
periments; and  a  comparison  of  the  Tables  will  show  that  a 
rough  correspondence  holds  in  the  ratios  sustained  between 
the  phases  of  visibility  and  the  adaptation  times  in  each  case. 

It  may  be  objected  that  the  colors  used  were  not  standard- 
ized. We  are,  however,  not  attempting  to  state  results  for 
standard  colors.  Our  sole  aim  is  to  show  correspondence  be- 
tween adaptation  and  fluctuation.  This  has  been  accomplished 
by  using  identical  colors  in  the  two  sets  of  experiments.  It 
could  have  been  done  no  better,  we  believe,  by  using  stand- 
ard colors. 

(c)  Combinations  of  stimulus  and  background  that  influence 
adaptation  time  correspondingly  influence  fluctuation  time.     ( i ) 
Fluctuation.     For  this  point  so  far  the  Masson  disk  has  been 
used.     From  all  the  colors  tried  as  background,  light  greenish 
blue  (Hering),  yellowish  green  (Milton- Bradley),  yellow  (Mil- 
ton-Bradley), orange  (Hering),  gray,  and  in  one  case  dark 
red  (Milton- Bradley)  were  selected  for  the  following  Tables. 
The  stimulus  strips  were  2   mm.  x  5  mm. ,  and  were  placed 
8  mm.  apart  along  the  radius.     They  were,  with  one  excep- 
tion, of  Hering  red. 

This  method  we  consider  very  unsatisfactory.  In  the  first 
place,  results  never  stand  out  so  clearly  with  the  Masson  disk 
as  when  the  system  is  at  rest:  judgments  are  difficult;  distrac- 
tions are  many,  and  gradations  of  intensity,  neither  so  constant 
nor  even  so  delicate,  can  be  obtained.  And,  secondly,  as  our 
disks  were  made,  the  stimulus  color  was  rendered  liminal  by 
mixing  with  the  color  of  the  background  rather  than  with  a 
gray  of  its  own  brightness.  If  we  take,  for  example,  a  red 
stimulus  upon  a  light  blue  background,  the  effect  obtained  was 
a  faintly  reddish  blue  upon  a  blue  background,  slightly  differ- 
ing from  it  in  brightness.  But  even  this  approximation  to  the 
desired  conditions  was  sufficient  to  vary  the  phase  of  visibility 
to  a  rough  correspondence  with  the  results  obtained  with  a 
similar  combination  of  stimulus  and  background  in  the  adapta- 
tion experiments. 

(2)  Adaptation.  Here,  likewise,  red  ^  full  intensity  disap- 
pears most  readily  upon  the  light  blue;  not  quite  so  readily 
upon  the  gray  used;  and  never  entirely  goes  into  the  back- 
ground, although  the  color  is  lost  periodically,  upon  the 
orange,  yellow,  and  yellowish  green.  Yellow  on  dark  red  is 
peculiarly  persistent. 

In  addition  to  the  combinations  here  used,  we  have  tried  a 
number  both  of  grays  and  of  colors,  and  are  satisfied  that 
whatever  alters  the  conditions  for  adaptation  correspondingly 
alters  the  conditions  for  fluctuation. 

(d)  Method  of  variation  of  areas,     (i)  Fluctuation.     This 


FLUCTUATION  AND  ADAPTATION. 


101 


TABLE  VI. 

Ga .   Combinations  of  stimulus  and  background  that  influence  adaptation 

time  correspondingly  influence  fluctuation 

time.    Fluctuation. 


Stimulus 

Background 

Vis. 

M.  V. 

Invis. 

M.  V. 

Vis.: 
Invis. 

Period 

Red,      2x5  mm. 

Light  Blue 

2.287 

.681 

3-362 

.631 

.6803 

5.649 

«            «      « 

Yellow 

4.609 

.1141 

2.78 

.600 

1.6856 

7.388 

«            a      « 

Orange 

4.709 

.1127 

2.936 

.663 

i.  600 

7.645 

«            «      « 

Yellowish  Green 

4.130 

1.161 

2-715 

.800 

1.521 

6.845 

180°  Engine  Gray  ) 

«            K      « 

+ 
180°  Velvet  Black  ) 

3-6i5 

1.076 

3-268 

i.i 

1.106 

6.893 

Yellow,  2x5  mm. 

Dark  red 

6.512 

1.  012 

3.887 

.912 

1.675 

10.399 

TABLE  VII. 

F.    Combinations  of  stimulus  and  background  that  influence  adaptation 

time  correspondingly  influence  fluctuation 

time.    Fluctuation. 


Stimulus 

Background 

Vis. 

M.  V. 

Invis. 

M.  V. 

Vis.: 
Invis. 

Period 

Red,      2x5  mm. 

Light  blue 

2.9 

•398 

3.164 

•58 

.969 

6.064 

«            «      « 

Yellow 

4-73 

-52 

2.96 

•4 

1.598 

7.69 

«            «      « 

Yellowish  Green 

4-4 

.717 

3-2 

•37 

1-375 

7.60 

180°  Engine  Gray  V 

tt            «      « 

180°  Velvet  Black  ) 

2.9 

.292 

2-95 

.628 

.983 

5-85 

method  was  tried  both  upon  the  Masson  disk  and  with  the  opal 
glass  plate  as  a  background.  The  results  in  both  cases  were 
unquestionable;  but,  as  before,  those  given  by  the  stationary 
system  were  much  the  more  satisfactory  and  much  the  more 
clearly  cut.  Because  of  this,  and  chiefly  because  the  disk  did 
not  permit  enough  variation  of  area,  the  Masson  disk  will  be 
omitted  from  further  consideration  in  this  paper. 

A  stimulus  was  obtained  upon  the  opal  glass  plate  by  light 
coming  from  a  bank  of  lamps  behind  it,  passing  first  through 
a  plate  of  frosted  glass,  then  through  the  opal  glass  itself. 
The  magnitude  of  the  stimulus  was  regulated  by  a  card-board 
diaphragm  behind  the  screen;  its  intensity,  by  varying  the 
distance  of  the  lamps,  also  by  means  of  a  curtained  window  in 
front.  This  photometric  arrangement  provided  a  very  sensitive 
means  of  obtaining  a  just  noticeable  stimulus.  After  the  ini- 
tial adjustment  was  made,  great  care  was  taken  that  the  illumi- 
nation of  the  background  should  remain  constant  throughout 
the  experiment. 


102 


FERREE  : 


TABI,B  VIII. 

F.    Method  of  variation  of  areas.    Fluctuation:  showing  inverse  varia- 
tion of  visibility  and  invisibility  with  increase  of  area. 


Area 

Vis. 

M.  V. 

Invis. 

M.  V. 

Vis.:  Invis. 

Period 

2x  2  mm. 

16.86 

4-83 

.8 

.28 

21.075 

17.66 

4x4    " 
6x  6    " 

12.93 
10.78 

3-53 
3-8 

I-I5 
2.9 

•38 
•83 

11.24 
3-71 

14.08 
13.68 

8x  8    " 

5.65 

i-53 

2.76 

•52 

2.23 

8.41 

10X10      " 

4-74 

1.32 

2-34 

•63 

2.023 

7.09 

12X12       " 

4.22 

•97 

2.72 

•57 

I-55I 

6.94 

14X14      " 

16x16    " 

1.29 

1.22 

3-2 
2.918 

.46 
•59 

1-359 
1.360 

!:& 

6x  6  cm. 

10X10      " 

3-73 
.81 

I.  II 

•23 

5-2 

9-65 

J7 

.717 
•073 

8.93 

10.46 

14x14    " 

.8 

•5 

29.46 

4.66 

.027 

30.26 

18x18    " 

•85 

•45 

40.25 

2.25 

.021 

40.10 

22X22      " 

1.4 

No  reappearance. 

For  F,  beginning  at  areas  ranging  from  10  cm.  x  10  cm. — 
14  cm.  x  14  cm.  in  the  different  records,  it  was  noticed  that 
only  the  edge  of  the  lower  left  hand  corner  and  left  side  reap- 
peared. 

The  results  of  this  Table  have  been  thrown  into  the  form  of 


1 1  M  M  n  1 1  hi  i  n  i  i>i  1 1 1 1  n  m  i  i  n  1 1  \rr\\\ 


CURVE  I. 

Curve  for  visibility.     TABLE  VIII.     Showing  decrease  of  visi- 
bility with  increase  of  area. 


FLUCTUATION   AND   ADAPTATION.  103 

a  curve.  The  dimensions  of  the  stimulus  are  laid  off  along  the 
abscissa,  millimeter  for  millimeter;  the  time  values  along  the 
ordinate,  on  the  scale  of  i  second  to  5  millimeters.  The  last 
and  more  horizontal  part  of  the  curve  for  visibility  represents 
the  reappearance  of  the  edge  of  the  left  side  and  the  lower  left 
hand  corner. 


CURVE  II. 

Curve  for  invisibility.*    TABLE  VIII.    Showing  increase  of  in- 
visibility with  increase  of  area. 


CURVE  III. 

Curve  for  visibility  :  invisibility.     TABLE  VIII.     Showing  de- 
crease with  increase  of  area. 

1  In  this  and  the  following  curves  invisibility  is  platted  as  a  negative 
quantity. 


104 


FKRREE : 


TABI,E  IX. 

Method  of  variation  of  areas.    Fluctuation:  showing  inverse  vari- 
ation of  visibility  and  invisibility  with  increase  of  area. 


Area 

Vis. 

M.  V. 

Invis. 

M.  V. 

Vis.:  Invis. 

Period 

2x  2   mm. 

7-3 

i  31 

2.122 

.466 

3-44 

9.422 

8x  8 

6-34 
4-53 

1*8 

i.  066 

2-94 
3-24 

.46 
-558 

2.16 
i-39 

9.28 

12X12 

4-23 

i-45 

3-75 

•53 

1.  12 

7.98 

16x16 

4-03 

•933 

3-9 

.708 

1.03 

7-93 

6x  6   cm. 

1-83 

•58 

5-675 

1.89 

'32 

7-505 

IOXIO 

1.19 

•436 

6.96 

2.8 

•17 

8.15 

16x16 

.61 

.1 

12.53 

3-3 

.048 

13-  X4 

26x26     _ 

•725 

•175 

25-36 

6.067 

.028 

26.085 

34x34 

•55 

.2 

25-13 

4-3 

.021 

25-68 

42x38 

.8 

Nc 

)  reappes 

irance. 

For  Ga,  with  an  area  of  10  cm.  x  10  cm.,  only  about  one 
third  of  the  area  covering  the  lower  left  hand  corner  reappeared. 
From  that  area  on,  the  part  reappearing  became  less  and  less, 
until  finally  there  was  no  reappearance  at  all. 

The  reappearance  of  the  lower  left  hand  corner  alone  in  the 
case  of  the  larger  areas  led  to  the  belief,  after  a  time,  that 
the  stimulus  was  stronger  in  this  region.  This  was  all  the 
more  probable,  because  the  window  was  somewhat  above  and 
to  the  right  of  the  observer,  thereby  illuminating  the  back- 
ground around  this  corner  slightly  less  than  the  rest  of  the 
field.  In  consequence,  this  part  of  the  stimulus  stood  out 
slightly  supfaliminally.  To  obviate  this  difficulty,  stimuli  of 
Hering  gray,  no.  27,  were  pasted  upon  engine-gray  card-board 
and  placed  behind  the  opal  glass  plate.  The  intensities  were 
easily  regulated  by  slight  changes  in  the  distance  of  the  plate 
from  the  card-board.  Since  both  stimulus  and  background 
were  now  seen  by  reflected  light,  the  former  inequality  of  rela- 
tion between  them  was  impossible.  The  rather  remarkable 
change  of  results  obtained  makes  it  worth  while  to  note  the 
following  Tables. 

TABI,E  X. 

F.    Method  of  variation  oj  areas.    Fluctuation:  showing  inverse  variation 
of  visibility  and  invisibility  with  increase  of  area. 


Area 

Vis. 

M.  V. 

Invis. 

M.  V. 

Vis.:  Invis. 

Period 

3x  3  mm. 
6x  6      " 

IOXIO        " 

2OX2O      " 

25x25    " 

6x  6    cm. 

IOXIO         " 

4-435 
3-372 
3-327 
3-3i 
2.962 

1-533 
I-I45 

.742 
•854 
•5 
-7i 

.427 

•333Nc 

4.285 
6.772 
11.562 

12.975 
14.36 
38.00 
>  reappez 

-835 
1.  218 

1-937 
1.84 
2.812 

•723 
irance. 

1-^35 
.498 
.287 

•255 
.206 
.0403 

8.72 
10.144 
14-889 
16.285 
17.322 
39-533 

FLUCTUATION  AND  ADAPTATION. 


105 


Curves  were  plotted  from  these  results  to  compare  with  those 
obtained  from  Table  VIII. 

It  will  be  noticed  that  this  series  began  with  an  area  of 
3  mm.  x  3  mm.  In  the  former  case,  it  was  2  mm.  x  2  mm. 


I  i  I  i  I  I  I  I  I  I  I  I  I  I  I  I  i  I  I 
CURVE  IV. 

Curve  for   visibility.     TABLE   X.     Showing   decrease   of  visi- 
bility with  increase  of  area. 


Curve  for  invisibility. 

visibility  with  increase  of  area. 


CURVE  V. 
TABLE    X.     Showing   increase  of  in- 


io6 


FERREE 


TABI,E  XI. 

Ga.   Method  of  variation  of  areas.    Fluctuation:  showing  inverse  variation 
of  visibility  and  invisibility  with  increase  of  area. 


Area 

Vis. 

M.  V. 

Invis. 

M.  V. 

Vis.:  Invis. 

Period 

4x  4  mm. 

4.466 

1.26 

1-853 

.406 

2.41 

6.319 

6x  6 

2-385 

.910 

2-934 

.828 

.812 

5-319 

12X12 

16x16 

2.000 
1.984 

JE 

5-257 
5.269 

i.  600 
1.768 

.380 
•376 

7.257 
7-253 

20X20 

1-453 

.615 

5-557 

2.050 

.261 

7.010 

25x25 

1.  122 

•321 

6.100 

1.538 

.183 

7.222 

4x  4    cm. 

•775 

.225 

12.583 

3-266 

.061 

13.358 

6x  6 

.716 

.200 

14.460 

4.216 

.049 

15.176 

10X10 

•632 

.197 

23.100 

3-500 

.027 

23-732 

12X12 

•590 

.284 

33-866 

7-445 

.017 

34-456 

14X14 

1*5 

N< 

)  reappe 

irance. 

The  following  curves  represent  the  results  of  the  preceding 
Table.     The  first  area  used  is  4  mm.  x  4  mm. 


TM  1  I  I  l  I 


trTL 


CURVE  VI. 

Curve  for  visibility.     TABLE  XI.    Showing  decrease  of  invisi- 
bility with  increase  of  area. 


M  \  n  n  1 1  p  u  NT  r  i  j  M 


CURVE  VII. 

Curve  for  invisibility.    TABLE  XI.    Showing  increase  of  invisi- 
bility with  increase  of  area. 


FLUCTUATION  AND  ADAPTATION. 
XII. 


107 


Ge.    Method  of  variation  of  areas.    Fluctuation:  showing  inverse  variation 
of  visibility  and  invisibility  with  variation  of  area. 


Area 

Vis. 

M.  V. 

Invis. 

M.  V. 

Vis.:  Invis. 

Period 

tx  3  mm. 
x  6      " 

10X10        " 
20X20        " 
25X25        " 

6x  6   cm. 

10X10         " 

5-43 
3-14 
2.469 
2.16 
1.185 
•7 

1.2 

1-5 

1-033 

•953 
.72 

•255 
•I33Nc 

3.192 

SL 

15-683 
18.342 

27-75 
>  reappei 

1.169 
•930 
-305 
4.016 
3-342 
5-85 
irance. 

1-732 
•730 
•305 
•137 
.064 
•025 

8.622 
7-44 
8-559 
I7-843 
15-75 
28.45 

XIII. 

8.    Method  of  variation  of  areas.  Fluctuation:  showing  inverse  variation 
of  visibility  and  invisibility  with  increase  of  area. 


Area 

Vis. 

M.  v! 

Invis. 

M.  V. 

Vis.:  Invis. 

Period 

3x  3  mm. 
6x  6 

4-683 
3-66 

1.26 
.686 

2.28 
2.26 

•633 
.626 

2.05 
1.619 

6.963 
5-92 

10x10 

2.876 

•557 

2-33 

•495 

1-234 

5.206 

16x16 
20x20 

2.521 
2.23 

•643 
.709 

!:i 

•556 
.668 

.980 
.796 

5.091 
5-030 

4x  4  cm. 
8x  8 

2.08 
2.288 

•625 
•594 

3-805 

•565 
1.047 

•697 
.600 

5.06 
6.093 

12X12 

2.03 

•523 

5-361 

1.230 

.378 

7-391 

16x16 

12.9 

2-385 

14.600 

18x18 

i-5 

Nc 

>  reappes 

irance. 

The  averages  of  the  visibilities  and  invisibilities  of  Tables 
X,  XI,  XII,  and  XIII  were  plotted  up  to  the  area  6  cm.  x 
6  cm.,  the  last  reappearance  recorded  for  /''and  Ge. 


CURVE  VIII. 

Curve  for  visibility.     Averaged  from  Tables  X,  XI,  XII,  and 
XIII.     Showing  decrease  of  visibility  with  increase  of  area. 


io8 


FKRREE : 


J  I  I   I  I  I  M  I  I  J  I  I 


CURVE  IX. 

Curve  for   invisibility.     Averaged  from  Tables  X,    XI,  XII, 
XIII.     Showing  increase  of  invisibility  with  increase  of  area. 

It  will  be  noticed,  in  the  above  Tables,  that  the  area  at 
which  fluctuation  ceases  has  been  decreased  to  one-fourth  in 
the  one  case  and  to  one- eighth  in  the  other.  This,  we  believe, 
is  due  solely  to  the  inequality  in  intensity  of  the  stimulus  ob- 
tained by  the  former  method,  for  it  will  be  observed  that  the 
area  at  which  the  stimulus  began  to  recur  in  parts  in  the  former 
Tables  nearly  coincides  with  that  at  which  fluctuation  ceased 
in  the  latter. 

It  will  be  seen,  also,  that  in  the  case  of  the  smaller  areas  the 
phases  of  visibility  have  been  decreased  and  the  phases  of  in- 
visibility increased.  Sufficient  explanation  for  this  result  can 
be  found,  most  probably,  in  the  different  conditions  for  adapta- 
tion present  in  the  two  cases.  It  will  be  well,  at  least,  to  point 
them  out. 

(a)  Although  of  no  greater  intensity,  the  stimulus  area  was 
more  sharply  defined  than  the  area  given  by  the  reflected  light. 
The  latter  was  somewhat  diffuse  and  spreading,  and  to  a  certain 
degree  gave  the  effect  of  a  larger  area.  This  slight  change 
would  be  appreciable  for  the  smaller  areas,  but  not  for  the 
larger.  (£)  The  side  of  the  opal  glass  used  for  the  back- 
ground in  the  former  case  was  polished  and  shining.  This  was 
trying  to  the  eyes  of  the  observer,  the  strain  relieving  itself  in 
increased  eye-movement  and  blinking.  The  side  used  in  the 
latter  case  was  dull  and  chalky,  and  produced  no  particular 
discomfort,  (c)  In  the  former  case,  the  minimal  difference  to 


FLUCTUATION  AND   ADAPTATION.  109 

be  adapted  out  was  between  a  bright  white  background  and  a 
still  brighter  stimulus.  In  the  latter  case,  it  was  between  a 
dull,  chalky  background  and  a  darker  stimulus.  Just  what 
effect  this  difference  would  have  on  adaptation  we  are  not  able 
to  state.  It  seems  reasonable  to  believe,  however,  that  the 
process  would  not  be  uniform  at  all  points  in  the  white-black 
series.  In  fact  records  were  obtained  indicating  that,  in  gen- 
eral, this  is  true;  unfortunately,  however,  they  were  made 
early  in  the  work,  and  were  not  arranged  for  a  particular  con- 
firmation of  results  under  these  precise  conditions.  As  nearly 
as  can  be  determined  from  them,  planned  as  they  were,  the 
process  is  more  rapid  at  the  extremes  of  the  white-black  series 
(indicated  by  the  shorter  phases  of  visibility  here  obtained) 
than  in  the  mid-region  of  neutral  grays.  The  stimulus  just 
noticeably  lighter  than  a  black  background,  too,  seems  to  give 
slightly  shorter  phases  of  visibility  than  a  stimulus  just  notice- 
ably darker  than  a  white  background.  It  will  be  understood 
that  these  results  are  not  intended  to  apply  to  adaptation  any 
further  than  for  the  obliteration  of  just  noticeable  differences. 
But  from  the  data  we  may  draw  the  very  general  conclusion 
that  the  value  of  just  noticeable  differences  for  adaptation, 
even  in  the  white-black  series,  depends  upon  the  sort  of  com- 
bination used. 

It  is  important  to  note  that  while  L,ange l  finds  approximate 
equality  in  the  periods  obtained  from  three  sense  departments, 
and  argues  therefrom  a  central  origin,  it  is  found  here  that  a 
change  of  conditions  so  slight  as  to  have  passed  unnoticed,  had 
not  the  results  demanded  investigation,  brings  a  wide  range  ol 
variability,  although  not  even  a  change  in  the  retinal  elements 
stimulated  is  involved,  i.  e. ,  both  series  of  combinations  of  stim- 
ulus and  background  are  in  the  white-black  series. 

Another  point  noticeable  in  these  records,  and  throughout 
the  work  in  general,  is  the  large  mean  variation.  This  is  es- 
pecially obvious  when  large  areas  are  used,  or  whenever  from 
any  cause  either  visibility  or  invisibility  approaches  infinite 
value.  It  is  due,  chiefly,  to  one  or  two  very  long  phases  of 
visibility  or  very  short  phases  of  invisibility,  or  conversely;  the 
phenomena  depending  upon  which  extreme  of  the  phase  varia- 
tion one  is  considering.  Slaughter2  believes  that  there  is  a 
connection  between  these  recurring  long  phases  of  visibility, 
obtained  with  stimuli  of  the  usual  order,  and  the  Traube-Her- 
ing  waves.  It  seems,  however,  much  more  probable  that  their 
immediate  condition  is  to  be  found  in  eye-movement.  In  un- 
steady fixation,  the  eye  oscillates,  /.  e.,  in  recovering  fixation, 

1  Philosophische  Studien,  IV,  390. 

2  Op.  cit. 


110  FERREE: 

it  overdoes,  swinging  to  the  other  side  and  back  again,  etc. 
Eye-movements  come  in  groups.  .  One  or  more  of  these  groups 
occurring  within  a  phase  of  visibility,  will  prolong  it  very 
much;  or  falling  within  a  phase  of  invisibility  will  shorten  it 
proportionately.  These  facts  are  brought  out  plainly  in  the 
records  for  eye-movement. 

That  extensive  voluntary  eye-movement  will  not  cause  the 
reappearance  of  the  faded-out  stimulus,  provided  sufficiently 
large  areas  are  used,  was  confirmed  for  Ga  and  S.  An  area 
of  opal  glass  30  cm.  square  was  made  just  noticeably  red  by 
light  transmitted  through  red  paper  covering  its  back.  Three 
fixation  points  were  made  the  apices  of  an  equilateral  triangle, 
circumscribed  about  the  centre  of  the  plate.  The  observer, 
seated  at  a  distance  of  98  cm. ,  allowed  the  color  to  adapt  out, 
and  then  shifted  his  eyes  along  the  sides  of  the  triangle  from 
fixation  point  to  fixation  point  in  order.  The  following  results 
were  obtained : 

Ga.     With  2  cm.  eye-movement  (in  each  direction)     .     .     .     .     .     . 

Slight  reappearance  at  edges  only. 

With  3.3  cm.  eye-movement Began  to  get  a  wash 

of  color  farther  in  from  the  edges. 

With  4  cm.  eye-movement Color  returned  a  little 

more  perceptibly  over  central  area. 

S  reported  no  change  at  all  until  4.9  cm.  of  eye-movement  in  each 
direction  were  reached.  Then  there  was  a  slight  wash  of  color,  pretty 
much  over  the  whole  area.  Had  he  observed  more  closely,  he  proba- 
bly would  have  noticed  the  changes  at  the  edges  sooner  than  this. 

The  intensity  of  the  stimulus  was  increased  considerably 
above  the  limen,  and  the  same  method  carried  out  with  similar 
results.  A  larger  area,  however,  had  to  be  used  with  the  same 
range  of  movement. 

These  facts  speak  strongly  against  innervation  as  the  cause 
of  the  reappearance  of  the  adapted  out  stimulus.  Much  more 
plausible  does  it  seem  that  restoration  comes  about  on  account 
of  actual  change  of  stimulation  of  the  adapted  elements.1 

(2)  Adaptation.  The  following  Tables  show  the  effect  of 
variation  of  area  for  recognized  adaptation  phenomena.  The 
combinations  of  stimulus  and  background  chosen  are  the  most 
favorable  for  intermittence.  Fluctuations  of  intensity  may  be 
had  from  any  combination,  but  complete  disappearances  take 
place  most  readily  with  those  here  selected.  The  eye-strain  in- 
volved and  the  consequent  unsteady  fixation  make  the  phenom- 
enon somewhat  difficult  to  obtain. 

1  Together  with  the  supplementary  process  mentioned  but  not 
specified  above. 


FLUCTUATION  AND  ADAPTATION. 


in 


TABUS  XIV. 

Ga.    Method  of  variation  of  areas.    Adaptation:  showing  inverse  varia- 
tion of  visibility  and  invisibility  with  increase  of  area. 


Stimulus 

Background 

Vis. 

M.  V. 

Invis. 

M.  V. 

Vis.: 
Invis. 

Period 

Red,    ix  5  mm. 

Blue 

No  disappearance. 

2x  5     " 

« 

4.64 

.98 

I-I75 

•325 

3.95 

5-815 

"        5^5     " 

« 

2-77 

.601 

4-19 

1.16 

.66 

6.96 

"         10X10       " 

«« 

1.4 

•34 

4-44 

1.24 

•31 

5-84 

"    2X35  " 

« 

1.68 

•58 

3-35 

i.  09 

•5 

5-03 

Hering  Gray, 
(no.  27)  ix  5  mm. 

(  Hering  Gray, 
(      (no.  33) 

No  disappearance. 

"       2X    5       " 

« 

27-9I5 

1-423 

•54 

.223 

51.694 

28.455 

"     5*  5     " 

"     IOXIO      " 

"   2X35  " 

« 
« 

« 

9-450 
6.787 
10.36 

2.617 
2.25 
3-07 

1.568 
2-275 
1.485 

•57 
•325 
.285 

6.057 
2.983 
6.976 

11.018 
9.062 
11.846 

TABI,E  XV. 

E.    Method  of  variation  of  areas.     Adaptation:  showing  inverse  varia- 
tion of  visibility  and  invisibility  with  increase  of  area. 


Stimulus 

Background 

Vis. 

M.  V. 

Invis. 

M.  V. 

Vis.: 
Invis. 

Period 

Red,  ix  5  mm. 

Blue 

9.5627 

2.66 

1-075 

•175 

8.895 

10.637 

"      3*  5     " 

4-237 

1.07 

i  .381 

.281 

3.067 

5.618 

"      7x10    " 

« 

1.878 

•255 

2-863 

•594 

.656 

4.741 

"       ix  5     " 
"       5^5     " 

Gray 
« 

3-542 
2.06 

1.071 
.480 

10.471 
8-55 

2.514 
1.71 

.3383 
.2409 

14-013 
10.  61 

F.     ADAPTATION  is  RENDERED  INTERMITTENT  CHIEFLY 

BY   EYE-MOVEMENT. 

"Even  in  fixation  intended  to  be  constant,  as  in  the  present 
investigation,  it  is  not  likely  that  the  eye  was  motionless  for  the 
eight  to  thirty  seconds  during  which  the  experiment  lasted,  as 
McAllister  has  recently  pointed  out  that  the  eye  is  seldom  at 
rest  for  one-ninth  of  a  second  continuously.  At  least  it  would 
be  most  unlikely  that  it  should  be  absolutely  at  rest  for  so  long 
a  period  as  twenty  seconds  and  then  move  unconsciously  at  the 
end  of  that  time."  * 

The  inference  contained  in  the  above  quotation  is  that,  if 
eye-movement  causes  relief  of  the  adapted  elements  suffi- 
cient to  bring  about  reappearance  when  complete  adaptation 
has  once  set  in,  disappearance  should  never  have  occurred; 

1W.  B.  Pillsbury:  The  Journal  of  Philosophy,  Psychology,  and  Sci- 
entific Methods,  II,  272.  Review  of  Zur  experimentellen  Kritik  der 
Theorie  der  Aufmerksamkeitsschwankungen,  by  B.  Hammer. 


112  FERREE : 

for  the  eye  is  moving  almost  continuously,  and  each  move- 
ment should  have  relieved  the  adaptation  that  had  taken 
place  previous  to  it.  Pillsbury  has,  however,  probably  not  con- 
sidered that  range  of  movement  is  a  factor  as  well  as  frequency. 
If  the  eye  moved  nine  times  a  second  with  sufficient  range, 
there  probably  never  would  be  noticeable  adaptation  for  small 
areas.  How  far  this  supposition  is  from  the  facts,  however,  is 
shown  by  our  results.  The  average  interval  between  move- 
ments extensive  enough  to  produce  a  noticeable  shift  of  the 
after-image  in  either  the  horizontal  or  vertical  plane,  viewed 
at  a  distance  of  i  meter  (and  certainly  smaller  movements 
could  scarcely  be  considered  to  bear  upon  the  point  in  ques- 
tion), ranges  from  £  sec.-2^  sec.  The  average  time  be- 
tween movements  shifting  the  after-image  2  mm.  in  either  plane 
ranges  from  i|-  sec. -2^  sec.;  4  mm.,  from  2^  sec. -48  sec.; 
6  mm.,  from  3-}-  sec. -96  sec.,  etc.  Now  it  will  be  remembered 
that  the  movements  in  each  plane  came  in  groups  of  two  and 
three,  so  that  these  intervals  in  most  cases  should  be  so  much 
multiplied.  According  to  this  account,  there  seems  to  be 
ample  opportunity  for  adaptation  to  take  place,  when  range  of 
movement  is  taken  into  consideration  as  well  as  frequency. 
Movements  as  small  as  those  referred  to  by  McAllister  would 
probably  produce  some  effect  in  delaying  adaptation;  but  com- 
plete restoration  before  the  stimulus  has  adapted  out,  or  reap- 
pearance after  it  has  disappeared,  is  doubtless  caused  by  groups 
of  movements  of  considerable  range. 

The  range  of  movement  required  will,  of  course,  depend 
upon  the  stimulus  area  used.  When  the  area  is  very  small, 
Pillsbury 's  inference  holds;  there  is  no  disappearance.  The 
restoration  afforded  by  eye-movement  here  cancels  the  effect  of 
adaptation  before  disappearance  takes  place.  This  is  one  of  the 
points  brought  out  by  our  method  of  areas.  On  the  other 
hand,  with  areas  varying  from  10  cm.  x  10  cm.  —  14  cm.  x 
14  cm.,  the  range  of  movement  for  our  observers  was  not 
great  enough  ever  to  produce  reappearance.  And  with  still 
larger  areas,  extensive  voluntary  movements  did  not  suffice 
even  to  revive  the  lost  sensation. 

We  do  not  assert  that  the  statement  quoted,  considered  as  a 
criticism  of  Hammer's  article,  is  not  well  grounded.  This 
article  is  chiefly  suggestive.  But,  on  the  other  hand,  it  is  only 
fair  to  remember  that  it  requires  positive  knowledge  to  over- 
throw as  well  as  to  establish  a  theory.  Both  statement  and 
criticism  should,  with  equal  care,  be  based  upon  ample  investi- 
gation. That  eye-movement,  blinking,  etc.,  interfere  with 
the  course  of  adaptation  is  not  a  recently  discovered  fact,  nor 
is  it  a  closed  subject.  Local  adaptation  still  presents  a  fruitful 
field  for  research. 


FLUCTUATION   AND   ADAPTATION.  113 

With  the  help  of  the  data  submitted  in  this  article,  we  trust 
that  no  intrinsic  difficulty  will  be  found  in  the  conception  that 
adaptation  is  rendered  intermittent  by  eye-movement.  Aside 
from  this,  too,  there  remains,  further  to  strengthen  the  theory, 
the  supplementary  factor  (not  yet  specified)  which  works  in 
conjunction  with  the  relief  afforded  by  a  shift  of  the  adapted 
elements  into  a  region  of  different  stimulation. 

For  the  investigation  of  eye-movement,  a  method  had  to  be 
selected  that  would  not  be  objectionable  to  the  observers,  and 
would  not  interfere  with  the  normal  course  of  the  phenomenon 
either  mechanically  or  by  way  of  distraction.  The  shifting  of 
the  negative  after-image  during  fixation  afforded  a  method 
somewhat  rough,  but  adequate  for  our  purpose.  Colored 
strips,  5  mm.  x  40  mm.,  were  used  as  stimuli.  They  were 
pasted  on  a  background  of  white  card-board,  with  the  shorter 
dimension  in  the  plane  in  which  the  eye-movement  was  to  be 
investigated.  The  determination  of  frequency  then  became 
merely  a  matter  of  recording  the  appearance  of  the  after-image 
to  the  right  or  left  or  above  or  below  the  stimulus,  separate 
series  being  made  for  both  planes.  For  the  determination  of 
range  of  movement,  narrow  strips  of  paper  of  the  same  bright- 
ness as  the  background  were  placed  successively  2,  4,  6,  8,  etc., 
mm.  from  the  stimulus,  and  only  those  movements  recorded 
that  caused  the  after-image  to  shift  to  or  beyond  these  strips. 
The  strips  were  so  inconspicuous  as  not  to  attract  the  eye  away 
from  the  fixation  point;  still,  it  was  not  difficult  to  judge  when 
the  after-image  reached,  or  passed  beyond  them.  They  were 
always  used,  also,  when  frequency  alone  was  to  be  determined, 
in  order  that  the  same  conditions  might  prevail  throughout. 
Some  periods  were  given  up  wholly  to  the  investigation  of  eye- 
movement  alone,  thus  determining  the  type  in  general;  while 
again  the  eye-movement  tracing  was  alternated  with  the  corre- 
sponding fluctuation  tracing,  in  order  to  establish  a  more  im- 
mediate connection  between  the  eye-movements  in  either  plane 
and  the  phases  of  visibility  and  invisibility  in  that  plane. 
Doubtless,  it  would  have  been  better  to  have  the  eye-movement 
recorded  while  the  fluctuation  was  in  progress,  could  this  have 
been  done  without  interfering  with  the  normal  course  of  the 
phenomenon.  As  it  was,  however,  enough  results  were  ob- 
tained to  render  conclusions  safe  as  to  the  type  of  the  observer. 

The  stimuli  in  both  the  eye-movement  and  fluctuation  ex- 
periments were  of  the  same  dimensions,  and  were  arranged  in 
precisely  the  same  way.  The  distance  of  the  observer,  through- 
out, was  i  meter.  The  color  of  the  stimulus  was  selected  with 
reference  to  the  vividness  of  the  after-image  for  the  particular 
observer.  Milton-Bradley  standard  green  was  used  for  S,  Ga, 


114  FERREE  : 

Ge,  while  red  of  the  same  make  gave  the  best  results  for  F. 
The  expression:  'stimulus  vertical*  will  be  used  when  the 
longer  dimension  of  the  strip  is  placed  in  the  vertical  plane, 
and  'stimulus  horizontal'  for  the  corresponding  arrangement  in 
the  horizontal  plane. 

The  following  results  were  obtained: 

(a)     Eye-movement  in  the  horizontal  and  vertical  planes . 
S.    Length  of 'observation:  97  sec. 

Stimulus  vertical. 
Strips  2  mm.  distant.     Recorded  all,  55 

All  reaching  to  strips  2  mm.  distant,  26 

«       «      4     «          «  H 

«  «          «       «      5     «          «  4 

Stimulus  horizontal. 

Strips  2  mm.  distant.     Recorded  all,  39 

All  reaching  to  strips  2  mm.  distant,  21 

((  ((  «  «  4  It  «  Q 

««««£««  2 

The  results  here  show  greater  range  and  greater  frequency  in  the 
horizontal  plane.  The  records  also  demonstrated  that  the  recovery 
was  quicker  in  this  plane. 

Ge.    Length  of  observation:  94  sec. 

Stimulus  vertical. 
Strips  2  mm.  distant.     Recorded  all,  46 

All  reaching  to  strip  4  mm.  distant,  \  40 

«          «          «      «      5     «          «  I4 

«  «  <C  «  g          ((  ft 

tt  «  «        <(     Jo      «  «  2 

Stimulus  horizontal. 

Strips  2  mm.  distant.     Recorded  all,  30 

All  reaching  to  strip  2  mm.  distant,  25 

"      "     4     "  14 

«c          «  «      «     5     «          «  0 

It  will  be  noticed  that  the  excess  of  range  in  the  horizontal  plane  in 
this  Table  is  considerably  greater  than  the  excess  of  frequency.  There 
is  quicker  recovery  also  in  the  horizontal  plane. 

Ga.    Length  of  observation:  96  sec. 

Stimulus  vertical. 

Strips  2  mm.  distant.     Recorded  all,  35 

All  reaching  to  strips  2  mm.  distant,  31 

"      "       4     "  2 

«          «  «      «       _     «          « 

Stimulus  horizontal. 

Strips  2  mm.  distant.     Recorded  all,  81 

All  reaching  to  strips  2  mm.  distant,  9 

«      •«       4     ««  o 

This  Table  shows  greater  frequency  in  the  vertical  and  greater  range 

in  the  horizontal  plane.     In  the  next  Table  the  experiments  for  range 

were  not  carried  out.      The  following  averages   for  frequency  were 

obtained: 


FLUCTUATION  AND  ADAPTATION.  115 

F.    Length  of  observation,  104  sec. 

Stimulus  vertical,  29 

horizontal,  17 

Towards  the  end  of  the  hour,  for  each  observer,  the  records  showed 
increase  of  eye-movement,  as  the  result  of  fatigue. 

(b)  Fluctuation  with  vertical  and  horizontal  arrangement  of  the 

stimulus. 

The  following  are  the  results  obtained  for  the  fluctuation 
experiments.  The  stimulus  was  rendered  liminal  by  being 
placed  behind  a  plate  of  opal  glass. 


TABLE  XVI. 

S.     Fluctuation  with  vertical  and  horizontal  arrangement  of  the  stimulus. 

Showing  how  arrangements  that  favor  maximal  and  minimal 

interference  with  adaptation  affect  the  phases  of 

visibility  and  invisibility. 


Stimulus 

Arrangement 

Vis. 

M.  V. 

Invis. 

M.  V. 

Vis.: 
Invis. 

Period 

Gray,  5x40  mm. 
«                  « 

Red, 
<«                 «( 

Green,           " 
«                 «( 

Yellow, 
«        «       « 

Vertical 
Horizontal 
Vertical 
Horizontal 
Vertical 
Horizontal 
Vertical 
Horizontal 

5-3545 
3-0733 
3-I52 

3.260 
5.0611 
4.642 

1.027 
.426 
.917 
•452 
i.  06 
.526 
.911 
.580 

2.4727 
2.7066 
1.641 
2.436 
1-493 
2.546 
1.588 
2.542 

.690 
.666 

•453 
.621 

•413 
.786 

•511 

•371 

2.165 

I-I35 
1.9207 
.968 
3-340 
1.2804 
3-187 
1-825 

7.8272 
5-7799 
4-793 
4-794 
6.479 
5.806 
6.6491 
7-1856 

TABLE  XVII. 

Ge.     Fluctuation  with  vertical  and  horizontal  arrangement  of  the  stimu- 
lus.    Showing  how  arrangements  that  favor  maximal  and 
minimal  interference  with  adaptation  affect  the 
phases  of  visibility  and  invisibility. 


Stimulus 

Arrangement 

Vis. 

M.  V. 

Invis. 

M.  V. 

Vis.: 
Invis. 

Period 

Gray,  5x30  mm. 

5x40  ;; 
5x50  II 

Green,  5x40  " 

Vertical 
Horizontal 
Vertical 
Horizontal 
Vertical 
Horizontal 
Vertical 
Horizontal 

3-873 
3-825 
4.006 
1.407 

4-353 
2.700 
4.400 
2.663 

-873 
-983 
1.293 
.268 
.822 
1.071 
1.054 
•49° 

2.32 

3-458 
2.526 

4-423 
2.700 
3-285 
4.981 
5.481 

•933 
1-075 
•733 
I-I53 
.868 

•778 
1.172 

I-I54 

1.669 
I-I35 
1-585 
.3181 
1.6123 
.8217 
-8833 
-4857 

6.193 
7.283 

6.533 
5-830 
7-053 
5-985 
9-38i 
8.144 

Il6  FERREE  : 

TABI,E  XVIII. 

Ga.     Fluctuation  with  vertical  and  horizontal  arrangement  of  the  stimu- 
lus.    Showing  how  arrangements  that  favor  maximal  and  min- 
imal interference  with  adaptation  affect  the 
phases  of  visibility  and  invisibility. 


Stimulus 

Arrangement 

Vis. 

M.  V. 

Invis. 

M.  V. 

Vis.: 
Invis. 

Period 

Gray,  10x50  mm. 

Vertical 

2-655 

.644 

6.422 

1-333 

•4135 

9.077 

«         «         (i 

Horizontal 

3-336 

•763 

5-545 

1.418 

.6016 

8.881 

Yellow,  2x40  " 

Vertical 

1.115 

3-223 

•915 

1.181 

7.030 

(t           «     « 

Horizontal 

6.96 

1.987 

2.45 

.801 

2.8408 

9-4! 

TABUS  XIX. 

F.     Fluctuation  with  vertical  and  horizontal  arrangement  of  the  stimulus. 

Showing  how  arrangements  that  favor  maximal  and  minimal 

interference  with  adaptation  affect  the  phases 

of  visibility  and  invisibility. 


Stimulus 

Arrangement 

Vis. 

M.  V. 

Invis. 

M.  V. 

Vis.: 

Invis. 

Period 

Red,    5x40  mm. 
«          «        «. 

Vertical 
Horizontal 

4.8685 
3.038 

1.29 
.646 

3-495 
3-753 

•775 
1.092 

1.392 
.809 

8-3635 
6.791 

(c)  Adaptation  with  vertical  and  horizontal  arrangement  of  the 

stimulus. 

That  the  same  arrangement  is  effective  with  stimuli  at  full 
intensity  was  confirmed  by  experiments  upon  Ga.  A  strip  of 
Hering  red,  5  mm.  x  30 mm.,  was  pasted  on  a  square  of  Hering 
light  blue,  20  cm.  x  20  cm.,  and  viewed  at  a  distance  of  2 
meters. 

TABI,E  XX. 

Ga.    Adaptation  with  vertical  and  horizontal  arrangement  of  the  stimulus. 

Showing  the  interference  caused  by  the  vertical 

and  horizontal  arrangements. 


Stimulus 

Arrangement 

Vis. 

M.  V. 

Invis. 

M.  V. 

Vis.: 
Invis. 

Period 

Red,  5x30  mm. 
«         <t         <« 

Vertical 
Horizontal 

2.24 
3-987 

.671 

.727 

4.014 
3-775 

.914 
.887 

•558 
1.056 

6.254 
7.762 

G.    CORRESPONDENCE  OF  FLUCTUATION  WITH  ADAPTATION 

IN   INDIRECT  VISION. 

(a)  Fluctuation.  In  the  fluctuation  experiments  in  indirect 
vision,  the  stimuli  were  rendered  liminal  by  the  use  of  the  opal 
glass  plate,  as  before.  The  observer  was  seated  at  a  distance 
of  i  meter  and  given  a  fixation  point  in  the  median  line.  It 


FLUCTUATION  AND  ADAPTATION. 


117 


will  be  noticed  in  these  results,  as  also  in  the  Tables  for  the 
method  of  variation  of  areas,  that  the  average  phase  of  visi- 
bility increases  slightly  at  the  end  of  the  Table.  The  reason 
is  that  in  each  tracing  the  phase  of  visibility  is  greatest  at  the 
beginning  and  decreases  considerably  towards  the  end.  Now 
in  the  last  series  of  the  Table  there  are  few  and,  at  the  very 
last,  no  phases  of  visibility  to  average  with  these  maximal  first 
phases;  consequently  the  curve  rises  a  little  at  the  lower  end. 
For  the  same  reason,  the  mean  variation  for  both  visibility  and 
invisibility  increases  towards  the  end  of  the  Table. 

The  results  obtained  are  given  in  the  following  Tables.  The 
points  to  be  noticed  are  the  effects  of  variation  of  area  and 
passage  of  stimulus  towards  the  periphery. 

TABI,E  XXI. 

Ga.     Correspondence  of  fluctuation  with  adaptation  in  indirect  vision. 

Fluctuation:  showing  the  effect  of  increase  of  area  and 

passage  of  stimulus  towards  the  periphery. 


Stimulus 

Distance  from 
fixation 

Vis. 

M.  V. 

Invis. 

M.  V. 

Vis.: 
Invis. 

Period 

Gray,   8x8   mm. 

o  cm. 

2-335 

.67 

2-38 

.486 

-985 

4-7I5 

« 

4 

1-527 

-383 

3-52 

•993 

•434 

5-047 

« 

8 

I-I3 

•345 

7-35 

2.212 

•153 

8.48 

« 

12 

i.  08 

•39 

8-73 

2-157 

.123 

9.81 

« 

20 

-566 

.1 

I3-38 

4-8 

.0423 

13.946 

« 

24 

•633 

.06 

34-53 

12. 

.018 

35-163 

6 

6    c 

n. 

0 

1-49 

.64 

6-33 

i-73 

•235 

7.82 

' 

4 

1.177 

•355 

8.411 

2.07 

.140 

9.588 

( 
t 

8 

12 

.628 
•625 

.214 
•J75 

10.685 
23-125 

2-514 
3-875 

.0588 
•0265 

11.312 
23-75 

< 

20 

-250 

•03 

43-25 

7.282 

.0057 

43-75 

24 

.2 

No  reappearance. 

TABI,E  XXII. 

F.     Correspondence  of  fluctuation  with  adaptation  in  indirect  vision. 

Fluctuation:  showing  the  effect  of  increase  of  area  and 

passage  of  the  stimulus  towards  the  periphery. 


Stimulus 

Distance  from 
fixation 

Vis. 

M.  V. 

Invis. 

M.  V. 

Vis.: 
Invis. 

Period 

Gray,   5x5   mm, 

o  cm. 

4.12 

•83 

2.41 

•50 

1.709 

6-53 

« 

8 

12 

1.671 
1.187 

•507 
•5 

4-5I4 
9.625 

.102 
2.28 

•370 

.122 

6.185 
10.812 

2O 

•433 

.123 

22.25 

4.82 

.0194 

22.683 

"        6x6    cm. 

O 

2-35 

.46 

4-77 

I-I3 

.492 

7.12 

8 

2.24 

•65 

14.04 

3-I58 

•1592 

16.28 

20 

1.5 

No  reappearance. 

Il8  FKRREE  : 

TABLE  XXIII. 

Ge.     Correspondence  of  fluctuation  with  adaptation  in  indirect  vision. 

Fluctuation:  showing  the  effect  of  increase  of  area  and 

passage  of  stimulus  towards  the  periphery. 


Stimulus 

Distance  from 
fixation 

Vis. 

M.  V. 

Invis. 

M.  V. 

Vis.: 
Invis. 

Period 

Gray,   6x6  mm. 

ocm. 

3-975 

1.  21 

4.187 

i-33 

•949 

8.162 

ii 

8   <k 

2.1875 

I.I 

8.4 

.24 

.2604 

10.5875 

« 

12 

1.128 

•285 

1.528 

3-045 

.09785 

12.656 

« 

20 

•79 

•25 

11.04 

.665 

•0715 

11.83 

** 

28 

•5 

.21 

11.728 

2.971 

.0426 

12.228 

tt 

34 

•375 

.1 

11.987 

2.446 

.0312 

12.362 

u 

43 

•5 

.2 

44.9 

9.62 

.0111 

45-4 

6x6    cm. 

0 

.62 

.22 

22.75 

4.64 

.0272 

22.97 

<t 

8 

•7 

.2 

42.6 

9.89 

•0151 

43-3 

« 

12 

i. 

• 

STo  rea 

ppeara 

nee. 

(b)  Adaptation.  That  stimuli  at  full  intensity  show  the 
same  law  of  inverse  variation  of  visibility  :  invisibility  from 
direct  vision  towards  the  periphery  was  verified  by  Ga.  Hering 
standard  red  upon  a  background  of  engine-gray  card-board 
(neutral  shade)  was  used. 


XXIV. 

Ga.     Correspondence  of  fluctuation  with  adaptation  in  indirect  vision. 

Adaptation:  showing  the  effect  of  passage  of 

stimulus  towards  the  periphery. 


Stimulus 

Distance  from 
fixation 

Vis. 

M.  V. 

Invis. 

M.  V. 

Vis.: 

Invis. 

Period 

Red,    8x8    mm. 

8cm. 

15.916 

3-25 

.2 

•033 

79.58 

16.116 

12 

6-575 

2.05 

1.683 

•475 

3-907 

8.258 

20 

4-293 

I-I3 

2.656 

•925 

i.  612 

6.949 

« 

24 

4.89 

1.05 

3-35 

1.151 

1.46 

8.24 

34 

3-2 

1.192 

5-5 

1.107 

.581 

8-7 

42 

2.9 

.766 

6.583 

1.208 

.440 

9-73 

« 

50 

2-33 

.86 

9.26 

2.32 

.240 

11.56 

« 

60 

•45 

No  reappearance. 

The  adaptation  times  in  indirect  vision  were  also  obtained 
for  the  same  stimulus.  The  background  in  this  case  was,  as 
before,  the  neutral  engine-gray  card-board.  The  time  required 
completely  to  adapt  out  the  stimulus  was  recorded.  This,  an 
adaptation  experiment  in  its  purest  form,  shows  that  the  time 
required  to  adapt  out  the  stimulus  decreases  as  we  go  towards 
the  periphery.  One  may  suggest  the  following  reasons  why 
this  decrease  should  occur: 

( i )  Decrease  of  the  retinal  stuff  towards  the  periphery. 
This  would  certainly  be  true  for  colored  stimuli. 


FLUCTUATION  AND   ADAPTATION.  1 19 

(2)  Since  the  eye  is  approximately  spherical  in  form,  and 
the  aperture  is  near  the  front  surface,  one  might  expect  less 
absolute  change  of  stimulation  area  towards  the  periphery  on 
account  of  eye-movement.     Experiments  to  test   the  matter, 
by  the  same  method  as  was  used  in  direct  vision,  showed  a 
marked  decrease  in  the  number  of  eye-movements  recorded  as 
the  stimulus  was  moved  towards  the  periphery.     Whether  this 
was  because  there  was  actually  less  range  of  movement  of  the 
after-image,  or  was  due  solely  to  the  greater  difficulty  of  ob- 
servation, we  are  not  able  to  state.     The  fact  that  there  was  a 
greater  decrease  in  range  than  in  frequency  would  seem  to  indi- 
cate that  the  effect  was  not  wholly  due  to  increased  difficulty 
of  observation. 

(3)  A  further  reason  will  be  discussed  when  we  deal  with 
the  fluctuation  of  after-images. 

The  observer  sat  with  eyes  closed  and  registration  key  up. 
The  drum  was  started.  At  a  signal  the  observation  was  begun 
and  the  key  pressed  down.  When  the  color  had  adapted  out, 
the  key  was  released.  The  results  were  as  follows: 

Ga.     Time  unit:  i  sec. 

Stimulus    8  cm.  from  fixation,  15-8 

12   "        "            "  9-7 

20   "        "            "  5-o 

"         28   "         "            "  4-8 

"         54   "         "            "  2.8 

II.     CUTANEOUS  STIMULI. 

(a)  Pressure.     Liminal  pressure  stimuli  were  applied  to  sev- 
eral observers,  but  no  fluctuations  were   experienced.     Very 
smooth  cork  wafers  supporting   minimal  weights  were  used, 
and  every  care  was  taken  to  insure  uniformly  distributed,  pure 
pressure  sensations. 

(b)  Electro- cutaneous.    L/iminal  electro-cutaneous  stimulation 
was  also  tried.     The  tip  of  the  tongue  was  selected  as  the  area 
most   sensitive   to   stimulation.     Strips  of  very  light  tin  foil 
(Christmas-tree  foil)  were  used  as  electrodes.     The  moist  sur- 
face of  the  tongue  readily  held  these  in  place.     There  was  no 
preliminary  sensation  of  pressure  or  contact.     The  observer 
was  not  even  able  to  tell  that  the  strips  were  in  place  when  the 
current  was  off.     A  Du  Bois-Reymond  sledge  was  chosen  as 
giving  the  most  easily  regulated  induction  current.     The  ob- 
server was  seated  in  a  distant  room,  his  head  fixed  in  a  head- 
rest, and  the  electrodes  clamped  in  place.    He  was  thus  isolated 
from  all  noise  and  distracting  influence.     An  electric   button 
was  near  his  hand  by  means  of  which  he  could  signal  to  the 
experimenter  and  thus  regulate  the  intensity  of  the  stimulus. 
With  care  just  noticeable  stimuli  were  easily  obtained;  but  no 


120  FERREE. 

fluctuations  of  intensity  could  be  detected,  although  repeated 
attempts  were  made  on  a  number  of  observers.  It  hardly  seems 
possible  that  failure  to  obtain  fluctuations  could  have  been  due 
to  faulty  conditions. 

We  submit  these  results  hoping  that,  when  they  have  been 
verified  elsewhere,  they  will  prove  as  decisive  to  others  as  they 
have  been  to  us.  We  trust  that  in  them  ample  evidence  has 
been  afforded  that  Lange  advanced  the  theory  of  fluctuation  of 
attention  upon  insufficient  data.  Indeed,  that  an  attempt  ever 
should  have  been  made  to  gather  together  these  discrete  sense- 
phenomena  under  the  head  of  'fluctuation  of  attention'  seems 
more  the  result  of  doctrinal  development  than  of  a  thorough- 
going consideration  of  the  phenomena  themselves. 


The  Intermittence  of  Minimal  Visual 
Sensations. 


THE  INTERMITTENCB  OF  MINIMAL  VISUAL 
SENSATIONS 

STUDIED  FROM  THE  SIDE  OF  THE  NEGATIVE  AFTER-IMAGE 


I 
THE  FLUCTUATION  OF  THE  NEGATIVE  AFTER-IMAGE 

By  C.  E.  FERREE,  Lecturer  in  Psychology  at  Bryn  Mawr  College, 
late  Assistant  in  Psychology  at  Cornell  University 


TABLE  OF  CONTENTS 

PAGE 

I.    Introduction  59 
i.     Statement  of  Problem 

a.  In  terms  of  the  writer's  previous  work  59 

b.  Historical  60 
ii.    Summary  of  the  writer's  experiments  on  the  fluctua- 
tion of  minimal  visual  sensations  63 

II.    The  Fluctuation  of  the  Negative  After-image  65 

i.     Consideration  of 

a.  Bering's  objections  to  eye-movement  as  a  causal  fac- 
tor, viz.  65 

1.  After  continued  fixation  of  a  stimulus,  rapid 
movement  of  the  eyes  away  from  and  back 
to    the  stimulus  does   not  produce  disap- 
pearance of  the  after-image  66 

2.  Movement  of    the  background  causes  the 
after-image  to  disappear;  hence  eye-move- 
ment can  possess  no  peculiar  power  to  cause 
disappearance  68 

3.  Near-lying  after-images  due  to  successive 
stimulations  do  not  fluctuate  together  69 

4.  Eye-movement  does  not  noticeably  affect  the 
after-image  when  observed  in  a  dark  field  of 
vision  71 

b,  Exner's  assertion  that  eye-movement  causes  the  dis- 

appearance of  the  after-image  by  distracting  from 
its  clear  perception  76 

ii.  Demonstration  of  causal  connection  between  eye-move- 
and  fluctuation,  as  against  the  theory  of  intrinsic  os- 
cillation 78 

a.  Results  in  general  78 

b.  Description  of  method  and  apparatus  81 

c.  Results  in  detail  81 

1.  Fluctuation  occurs  only  within  a  limited 

range  of  after-image  areas  82 

2.  Whatever  renders  fixation  unsteady  increases 

the  fluctuation  and  decreases  the  duration 

of  the  after-image  87 


INTBRMITTBNCB  OF  MINIMA!,  VISUAL  SENSATIONS        59 

3.  Form  of  stimulus  affects  frequency  of  fluctua- 

tion and  duration  of  after-image,  in  a  de- 
gree roughly  proportional  to  its  effect  on 
range  and  frequency  of  eye-movement  97 

4.  Arrangement  of  stimulus  with  reference  to 

direction  of  greatest  eye-movement  affects 
frequency  of  fluctuation  and  duration  of 
after-image  101 

5.  The  results  under  i,  3  and  4  can  be  roughly 

paralleled  by  the  use  of  voluntary  eye- 
movement  to  cause  fluctuation  103 

6.  Increased  time  of  stimulation  increases  fluc- 

tuation of  after-image  108 

7.  Observers   most  sensitive    to  the    methods 

used  to  disturb  fixation  showed  the  widest 
range  of  variability  of  fluctuation  and 
duration  112 

8.  Practice  in  fixation  decreases  frequency  of 

fluctuation  and  increases  duration  112 

iii.     How  does    eye-movement  cause   fluctuation  and  de- 
crease duration  of  after-image?  112 

a.  Theories  of  Fechner,  Helmholtz  and  Fick  and  Giir- 

ber  inadequate  112 

b.  Explanation  found  in  effect  of  eye-movement  upon 

streaming  phenomenon  114 

1.  Description  of  phenomenon  114 

2.  Physiological  interpretation  116 

(a)    Not  entoptic  or  circulatory  116 

(£)     Probably  a  streaming  of  some  retinal 
material  capable  of  affecting  the 
j  visual  processes  116 

3.  Effect  of    streaming  on   after-image    same 

with  closed  and  open  eyes  117 

4.  Effect  of  streaming  on  flight  of  colors  120 

5.  Dependence  of  streaming  upon  eye-move- 

ment 122 

c.  Final   explanation  of   effect  of   eye-movement    on 

fluctuation  and  duration  of  after-image  123 

III.     Conclusion  and  restatement  of  thesis  129 

I.     INTRODUCTION 

This  paper  is  a  continuation  of  a  study  published  in  this 
Journal  in  January,  1906.*  In  that  article  the  fluctuation  of 
minimal  cutaneous  and  minimal  visual  stimuli  was  discussed. 
The  absence  of  fluctuation  for  cutaneous  stimuli  was  demon- 
strated by  a  series  of  experiments,  upon  a  number  of  observers, 
with  liminal  pressure  and  electrical  stimuli.2  The  fluctuation 

1C.  E.  Ferree:  An  Experimental  Examination  of  the  Phenomena 
usually  Attributed  to  Fluctuation  of  Attention  :  XVII,  81. 

2 This  part  of  the  work  has  recently  been  repeated  and  extended  by 
L.  R.  Geissler  (Fluctuations  of  Attention  to  Cutaneous  Stimuli,  this 
Journal,  XVIII,  1907,  309),  and  the  previous  results  are  confirmed. 
On  the  score  of  method  it  may  be  noted  that  a  very  thin,  narrow  strip 
of  high-grade  wrapping  foil  does  better  service  than  Christmas  tree 
foil,  and  that  the  tongue  should  not  be  protruded,  but  allowed  to  lie 


60  FKRREE 

of  minimal  visual  stimuli  was  explained  as  a  phenomenon  of 
adaptation.  The  thesis  was  maintained,  first  that  adaptation 
is  rendered  intermittent  chiefly  through  the  influence  of  invol- 
untary eye-movement;  and  secondly  that  eye-movement  inter- 
feres with  adaptation  in  two  ways:  it  reduces  the  time  of  stimu- 
lation, and  by  shifting  the  retina  into  a  region  of  different 
stimulation,  it  causes  the  restoration  of  the  adapted  elements. 
The  first  of  the  effects  just  mentioned  is  obvious;  but  the 
second  demands  explanation.  That  eye-movement  does  restore 
the  adapted  retina  can,  in  the  present  state  of  our  knowledge 
of  the  phenomenon,  scarcely  be  questioned;1  but  why  a  single 
eye-movement,  consuming  but  a  fraction  of  a  second,  or  even 
a  group  of  eye-movements,  is  able  to  restore  a  color  or  bright- 
ness that  required  a  much  longer  time  for  adaptation,  is  not 
readily  understood  and  needs  to  be  investigated.  Both  the  fact 
and  its  explanation  have  been  the  subject  of  much  discussion  in 
the  history  of  visual  theories.  This  discussion  will  be  touched 
upon  only  briefly.  It  is  not  our  purpose  here  to  give  a  detailed  ac- 
count of  visual  theories.  That  will  be  attempted  in  a  later  paper. 
The  object  at  this  time  is  briefly  to  state  such  phases  of  visual 
theory  as  will  be  sufficient  to  introduce  our  problem.  We  find, 
in  general,  two  main  types  of  theory:  the  one  represented  by 
Fechner,2  Helmholtz,8  Kick  and  Giirber,  etc.,  called  the  theory 
of  fatigue;  and  the  other  represented  by  Plateau,4  Hering,5 
G.  B.  Muller,6  and  others,  usually  called  the  theory  of  antag- 
onistic visual  processes,  or,  when  the  after-effect  of  stimulation 
is  more  especially  regarded,  the  oscillatory  theory. 

naturally  upon  the  floor  of  the  slightly  opened  mouth,  with  its  edges 
pressing  lightly  against  the  lower  teeth  and  lip. 

1See  Fechner,  Pogg.  Ann.,  XUV,  1838,  525;  Helmholtz,  Physiol. 
Optik,  1896,  510;  Pick  and  Giirber,  v.  Graefe's  Archiv,  XXXVI,  2, 

1890,  246;    Hess,  v.  Graefe's   Archiv,   XI/,  I,  1894,    274;  MacDougall, 
Mind,  XI,  1902,  316;  XII,  1903,  289;  C.  E.  Ferree,  this  Journal,  XVII, 
1906,  79-121. 

2  Pogg.  Ann.,  XLIV,  1838,  221,  513;  XI/V,  1838,  227;  L,  1840,  193,  427. 
The  theory  was  conceived  earlier  by  Scherffer  (Abhandlung  von  den 
zufalligen  Farben,  Wien,  1765;  also  Journal  de  Physique  de  Rozier, 
XXVI,  175,  273),  who  explained  the  negative  after-image  by  the 
diminished  sensitivity  of  the  fatigued  retina. 

8Miiller's  Archiv  f.  Anat.  u.  Physiol.,  1852,  461;  Pogg.  Ann., 
LXXXVII,  45;  Philos.  Mag.,  (4)  IV,  519;  Physiol.  Optik,  1896. 

4  Ann.  de  Chimie  et  de  Phys.,  1,111,  1833,  386;  I/VTI,  1835,  337;  Pogg. 
Ann.,  XXXII,  1834,  543.  More  fully  in  Essai  d'une  the"orie  ge"ne"- 
rale,  etc.,  Mem.  de  1'Acad.  de  Belgique,  VIII,  1834.  For  a  still  ear- 
lier form  of  the  theory,  see  de  Godart,  Journal  de  Physique  de  Rozier, 
VIII,  1776,  i,  269. 

6Zur  Ivehre  vom  L,ichtsinne,  1874;  v.  Graefe's  Archiv,  XXXVII,  3, 

1891,  i;    XXXVIII,  2,  1892,  252. 

6Zur  Psychophysik  der  Gesichtsempfindungen,  1897  (off-printed 
from  Zeitschrift  f.  Psychologic,  X,  1896,  i,  321;  XIV,  1897,  i,  161). 


INTKRMITTENCB   OF   MINIM AI,  VISUAL   SENSATIONS        6 1 

The  oscillatory  theory,  in  general,  does  not  attribute  to  eye- 
movetnent  any  direct  influence  upon  adaptation.  As  formulated 
by  Plateau,  1833-35,  it  was  primarily  intended  to  explain  the 
occurrence  of  the  positive  and  negative  after-images,  the  fluc- 
tuation of  the  negative  after-image,  and  the  flight  of  colors. 
Every  light  stimulation  arouses  two  processes  in  the  retina: 
the  one  corresponding  to  the  positive  visual  impression,  and 
the  other  to  the  negative  after-image.  When  the  eye  has  been 
stimulated  by  light  and  the  stimulus  is  removed,  the  retina 
attains  to  its  normal  state  only  after  a  series  of  oscillations  be- 
tween these  opposing  processes.  When  the  after-effect  of 
stimulation  is  observed  in  an  illuminated  field  of  vision,  the 
positive  phases  of  the  oscillating  processes  are  obscured  by  the 
general  illumination.  Hence  we  have  the  phenomenon  of  the 
negative  after-image  and  its  fluctuation  (the  phases  of  invisi- 
bility of  the  negative  after-image  corresponding  to  the  recur- 
rence of  the  obscured  positive  after-image).  When,  however, 
the  observation  is  made  in  a  darkened  field  of  vision,  all  the 
changes  are  noticeable.  The  changes  thus  observed  in  after- 
images of  certain  brightly  luminous  stimuli  have  been  denomi- 
nated the  'flight  of  colors.' 

The  phenomenon  of  adaptation  was  not  discussed  in  this 
early  form  of  the  theory.  It  was  taken  into  account  only 
when  the  cardinal  features  of  Plateau's  theory  (the  antagonistic 
nature  of  the  visual  processes  and  the  oscillatory  after-effect  of 
stimulation)  were  later  made  the  basis  of  a  group  of  visual 
theories,  of  which  the  Hering  theory  may  be  taken  as  the  type. 
These  theories  were  called  upon  to  explain  not  only  how  the 
eye  becomes  adapted  to  a  given  stimulus,  but  also  how,  when 
once  adapted,  it  is  restored  to  its  normal  state.  In  general, 
they  account  for  the  restoration  of  the  adapted  retina  by  the 
conception  of  antagonistic  processes :  every  visual  process  car- 
ries with  it  its  corrective  process.  All  factors  extrinsic  to  the 
processes  themselves  are  declared  to  be  unimportant.  It  is 
said,  e.  p.,  that  the  eye  does  not  become  permanently  adapted 
to  a  given  set  of  stimuli,  or  condition  of  illumination,  first,  be- 
cause of  the  tendency  toward  correction  inhering  in  the  nature 
of  the  opposing  processes,  and,  secondly,  because  the  stimula- 
tion of  the  eye  is  continually  changing,  owing  to  changes  in 
position  of  body  and  head,  movement  of  eyes,  blinking,  etc. 
But  these  latter  factors  are  on  no  account  to  be  considered  as 
exerting  a  direct  influence  on  the  retinal  processes.  They 
merely  give  the  corrective  tendency,  inhering  in  the  sets  of 
processes,  a  better  chance  to  operate. 

It  will  be  seen  that  the  above  mentioned  provision  explains 
the  relief  of  adaptation  only  in  a  general  way,  and  also  inade- 
quately. For,  without  taking  into  consideration  extrinsic  in- 


62  FKRREE 

fluences,  it  cannot  account  for  the  comparatively  large  measure 
of  restoration  brought  about  in  the  short  intervals  of  relief 
from  stimulation  afforded  by  the  involuntary  eye-movements 
which  occur  under  the  conditions  of  normal  fixation,  or  by 
voluntary  eye- movements,  of  short  duration,  away  from  the 
stimulus  and  back  again.  The  intervals  are  much  too  short 
for  correction  to  take  place,  to  such  a  degree,  of  its  own 
accord. 

As  these  disturbances  in  adaptation  cannot  be  explained, 
they  are  denied  to  be  of  any  considerable  importance.  Invol- 
untary eye-movement  under  the  conditions  of  normal  fixation, 
or  voluntary  eye-movement  away  from  the  stimulus  and  back 
again,  is  said  to  exert  no  noticeable  effect  upon  the  restoration 
of  the  adapting  stimulus,  or  upon  its  obverse  aspect,  the  nega- 
tive after-image.1  The  interval,  during  which  the  retina  is 
shifted  away  from  a  given  stimulus,  must  in  point  of  time 
alone  account  for  the  progress  it  has  made  towards  regaining 
its  normal  condition. 

On  the  other  side,  however,  the  representatives  of  the  theory 
of  fatigue  attribute  a  direct  influence  to  eye-movement  in  re- 
storing the  stimulated  retina  to  its  normal  state.  This  influence 
is  inferred,  more  particularly,  from  the  effect  of  eye-movement 
upon  the  negative  after-image.  The  argument  is  as  follows. 
A  voluntary  eye-movement,  or  a  noticeable  involuntary  eye- 
movement,  causes  the  after-image  momentarily  to  disappear. 
The  negative  after-image  may  be  taken  as  the  index  of  retinal 
fatigue ;  hence  whatever  is  able  thus  profoundly  to  disturb  the 
cause  of  the  after-image  must  also  function  in  the  recovery  of 
the  fatigued  retina.2 

According  to  Fechner  (opp.  citt.)  eye-movement  causes  the 
after-image  to  disappear  because  of  mechanical  disturbances  in 
vascular  and  nervous  influences  on  the  retina:  temporary  vas- 
cular congestion,  etc.  Helmholtz 8  says  that  eye-movement 
causes  the  after-image  to  disappear  by  producing  changes  in 
the  illumination  of  the  retina.  Both  writers,  apparently,  have 
practically  disregarded  the  effect  of  eye-movement  upon  the 
total  duration  of  the  after-image;  although,  as  will  be  shown 
later,  this  must  be  considered  a  much  more  important  factor  in 
a  study  of  the  restoration  of  the  adapted  retina  than  are  the 
momentary  disappearances  or  fluctuations  of  the  after-image. 

Of  the  more  recent  writers,  Fick  and  Giirber  contend  that 
eye-movement  changes  the  lymph-stream  in  a  way  that  facili. 
tates  the  removal  of  the  fatigue  products  and  the  delivery  of 

1  Hering:  opp.  citt. 

8  See  especially  Fick   and  Gurber,  v.  Graefe's  Archiv,  XXXVI,  2, 
1890,  246. 
8  Physiol.  Optik,  1896,  510. 


INTERMITTKNCE  OF   MINIMAL  VISUAL  SENSATIONS        63 

new  material  to  the  fatigued  areas.  Hering,1  replying  to  Kick 
and  Giirber,  denies  to  eye- movement  any  peculiar  power  to  re- 
lieve adaptation.  He  asserts  that  movement  of  the  field  of 
vision,  for  example,  answers  the  purpose  equally  well.  Hess 
(op.  cit.}  contends  that  an  adapted  stimulus,  steadily  fixated, 
is  not  recovered,  but  does  not  explain  how  eye-movement  re- 
stores the  adapted  retina.  Finally,  MacDougall  (opp.  dtt.) 
explains  the  effect  of  eye- movement  upon  the  reappearance 
of  minimal  visual  stimuli  on  the  basis  of  innervation. 

This  is  the  condition  in  which  we  find  the  problem  at  the 
present  time.  The  oscillatory  theory  makes  no  provision  for 
any  noticeable  effect  of  eye-movement  upon  adaptation,  nor 
can  it  explain  the  after-image  results  which  we  ourselves  have 
obtained.  Fechner  and  Helmholtz  ascribe  to  eye-movement  a 
direct  influence  upon  adaptation,  but  their  hypothesis  as  to  the 
way  in  which  this  effect  is  produced  can  be  shown  to  be  unten- 
able. MacDougall's  position  has  already  been  discussed  by 
the  writer;2  while  Hering,  as  will  be  shown  later,  did  not  carry 
his  observations  far  enough.8 

While,  however,  the  literature  does  not  furnish  a  satisfac- 
tory solution  of  the  problem,  it  strongly  suggests  a  method  of 
investigation.  It  is  evident  that  we  cannot  adequately  study 
recovery  while  the  stimulus  is  acting.  We  can  only  note  the 
coincidence  of  eye-movement,  or  what  not,  with  recovery. 
What  goes  on,  in  the  small  interval  for  recovery  afforded  by 
a  single  eye-movement,  defies  observation  or  experimental 
analysis.  Fortunately,  however,  the  after-effect  of  stimulation 
affords  an  easy  and  obvious  point  of  attack.  Here  we  can  study 
recovery  in  isolation,  and  may  hope  to  determine  the  factors 
that  influence  it :  the  factors  that  cause  the  fluctuations  and 
affect  the  duration  of  the  after-image.  In  accordance  with 
this  plan,  a  series  of  experiments  on  the  negative  after-image 
was  begun  in  the  Cornell  University  laboratory  in  the  spring 
of  1904.  The  results  of  these  experiments  will  form  the  sub- 
ject-matter of  this  and  the  following  papers. 

The  material  may  be  classified  under  the  following  heads  : 
I.  Relation  of  the  negative  after-image  to  adaptation;  II. 
Fluctuation  of  the  negative  after-image;  III.  Duration  and 
fluctuation  of  the  negative  after-image  with  reference  to  its 
bearing  upon  the  intermittence  of  minimal  visual  stimuli. 
This,  the  logical  order  of  treatment  will,  however,  be  changed 
for  the  sake  of  convenience  of  discussion.  The  determination 

1  v.  Graefe's  Archiv,  XXXVII,  3,  1891,  22. 

8  This  Journal,  XVII,  1906,  89. 

8  Pick  and  Giirber's  hypothesis,  although  too  indefinite  and  too 
speculative,  seems  to  be  the  most  promising  of  any  of  the  historical 
hypotheses.  It  will  be  discussed  later. 


64  FERREE 

of  the  relation  of  the  negative  after-image  to  adaptation  de- 
pends, in  part,  upon  the  results  of  the  two  succeeding  inqui- 
ries, and  can  therefore  be  most  conveniently  taken  up  at  the 
end  of  the  series.  We  shall,  accordingly,  begin  with  a  discus- 
sion of  the  cause  of  the  fluctuation  of  the  after-image,  and  of 
the  factors  influencing  its  duration.  In  terms  of  theory,  these 
must  constitute  the  factors  that  work  for  the  restoration  of  the 
stimlated  retina;  for  whatever  theory  is  held  of  the  adaptation 
and  after-image  phenomenon, — whether  it  be  ascribed  to  fa- 
tigue, to  antagonism  of  retinal  processes,  or  what  not,— the  factors 
that  work  against  the  after-image  operate  to  restore  the  stimu- 
lated retina  to  its  normal  condition.  To  make  our  position 
more  secure,  however,  we  shall  report,  in  a  second  paper,  adap- 
tation experiments  which  show  that  the  experimental  varia- 
tions that  increase  the  frequency  of  fluctuation  of  the  after- 
image and  decrease  its  duration  increase  the  time  required  for 
a  stimulus  to  adapt;  and,  conversely,  that  the  devices  that  de- 
crease the  frequency  of  fluctuation  and  increase  the  duration  of 
the  after-image  increase  the  time  required  for  a  stimulus  to 
adapt.  The  tests  thus  established  will  then  be  applied  to  the 
fluctuation  of  liminal  stimuli.  It  will  be  shown  that  whatever 
increases  the  fluctuation  of  the  after-image  and  decreases  its 
duration,  increases  the  phase  of  visibility  and  decreases  the 
phase  of  invisibility  of  the  liminal  stimulus;  and  conversely, 
that  whatever  decreases  the  fluctuation  of  the  after-image  and 
increases  its  duration,  decreases  the  phase  of  visibility  and  in- 
creases the  phase  of  invisibility  of  the  liminal  stimulus;  as 
should  be  the  case,  if  the  fluctuation  of  liminal  stimuli  is  an 
adaptation  phenomenon.1  Thus  the  chain  of  identification  will 
be  rendered  complete.  The  intermittence  of  minimal  visual 
stimuli  will  have  been  made  to  answer  to  the  tests  for  adapta- 
tion, from  both  its  obverse  and  its  reverse  sides.  Moreover, 
in  the  progress  of  the  work,  an  answer  will  be  found  to  the 
question  how  eye-movement  is  able  to  relieve  adaptation. 

The  whole  course  of  the  work  which  we  have  undertaken  on 
the  fluctuation  of  minimal  visual  stimuli  may  be  summarized 
as  follows.  First,  an  examination  of  the  phenomenon  was 
made  for  the  ascertainment  of  its  possible  causal  factors.  These 
were  found  to  be  disturbances  in  accommodation ;  adaptation, 
which  is  found  to  be  intermittent  with  normal  fixation ;  fluc- 
tuation of  attention;  and  physiological  disturbances  in  the  vis- 
ual centre  due  to  the  function  of  other  brain  centres,  such,  for 

1  This  result  follows  directly  from  the  adaptation  experiments  just 
referred  to.  For  whatever  increases  the  fluctuation  and  decreases  the 
duration  of  the  negative  after-image,  increases  the  time  required  for 
a  given  stimulus  to  adapt  (the  phase  of  visibility  when  the  stimulus 
is  liminal),  and  so  on. 


INTERMITTENCE   OF   MINIMAL  VISUAL  SENSATIONS        65 

example,  as  the  respiratory  and  circulatory  centres.  Secondly, 
we  eliminated,  by  experimental  process,  all  of  these  factors  with 
the  exception  of  adaptation.  Thirdly,  we  have  been  able  to 
identify  fluctuation  with  intermittent  adaptation  from  both  its 
obverse  and  its  reverse  sides.  And  fourthly,  we  have  deter- 
mined the  factors  that  disturb  adaptation. 

II.    THE  FLUCTUATION  OF  THE  NEGATIVE  AFTER-IMAGE. 

There  is  a  strong  interest  in  the  fluctuation  of  the  negative 
after-image,  independently  of  its  bearing  upon  our  special 
problem.  It  is  generally  recognized  as  one  of  the  important 
problems  in  psychological  optics,  and  one  not  as  yet  adequately 
taken  into  account  by  visual  theories.  We  find,  for  example, 
Plateau,  Aubert,  Hering,  Ebbinghaus  and  others  holding  that 
periodicity  is  grounded  in  the  nature  of  the  after-image  process; 
the  followers  of  Fechner  and  Helmholtz  contending  for  acci- 
dental influences  of  various  kinds  which  operate  upon  the  fa- 
tigued retina;  and  Exner  maintaining  that  eye-movement 
causes  the  after-image  to  disappear,  because  it  distracts  from 
clear  perception.  The  question,  then,  is  still  open.  The  evi- 
dence is  such  that,  unless  prejudiced  in  favor  of  some  particular 
explanation,  one  cannot  subscribe  to  any  without  further  in- 
vestigation. Thus,  von  Kries,  writing  in  1905,  testifies  to  the 
lack  of  decisive  results  as  follows :  "Die  Frage,  ob  das  Schwin- 
den  einer  lokalen  Umstimmung  sich  iiberhaupt  in  dieser  Form 
eines  allmahlichen  Abklingens  vollziehe,  ist  (ohne  messende 
Versuche)  viel  diskutiert  und  mehrfach  in  verschiedenem 
Sinne  beantwortet  worden."  1 

i.     Hering  and  Exner. 

In  our  consideration  of  the  various  theories,  those  which 
deny  causal  relation  between  eye- movement  and  fluctuation, 
Hering's  and  Exner's,  will  be  examined  first.  The  hypotheses 
which  seek  to  explain  the  effect  of  eye- movement  on  fluctua- 
tion, Fechner' s,  Helmholtz'  and  Fick  and  Gurber's,  will  be 
deferred  until  a  later  point  in  the  inquiry.  It  is  convenient  to 
begin  with  Hering. 

a.  Hering's  discussion  of  the  effect  of  eye- movement  on  the 
fluctuation  of  the  negative  after-image  (1891)  grew  out  of  a 
controversy  regarding  the  effect  of  eye- movement  upon  the 
restoration  of  the  fatigued  retina  to  its  normal  condition.  In 
this  discussion,  he  has  not  kept  the  two  subjects  formally  sep- 
arated, so  that  there  must  be  more  or  less  cross-reference  be- 
tween them  in  our  review;  although  the  centre  of  interest  for 
us,  at  this  stage,  is  the  fluctuation  of  the  negative  after-image. 

1  Nagel:  Handbuch  d.  Physiol.  des  Menschen,  III,  216. 
JOURNAL,— 5 


66  FBRREE 

The  whole  question  of  the  influence  of  eye-movement  on  the 
visual  processes  was  raised  by  the  representatives  of  the  theory 
of  fatigue.  They  sought  to  explain  first  why  the  eye,  fatigued 
by  a  particular  stimulus,  recovers  as  quickly  as  it  does;  and, 
secondly,  why  it  does  not  become  progressively  fatigued  by 
light  stimulation  in  general  during  the  twelve  hours  or  more 
of  its  exposure  to  light  in  the  course  of  a  day.  The  explana- 
tion was  given  chiefly  in  terms  of  the  changes  brought  about 
in  the  fatigued  retina  by  eye-movement.  As  has  been  stated 
above  (p.  62),  Fechner  asserted  that  these  changes  are  of  the 
nature  of  mechanical  disturbances  in  vascular  and  nervous  in- 
fluences; while  Helmholtz  attributed  them  to  the  more  or  less 
continual  changes  in  the  illumination  of  the  retina  due  to  eye- 
movement  in  connection  with  blinking,  frowning,  etc.  Obvi- 
ously, neither  hypothesis  is  adequate  to  explain  the  facts  in 
question. 

Fick  and  Giirber  (1890),  taking  up  the  problem  at  this 
point,  were  concerned  first  to  furnish  a  more  extended  experi- 
mental demonstration  of  the  fact  that  eye-movement  is  effective 
to  relieve  the  fatigued  retina;  and  secondly  to  explain,  more 
adequately  than  had  been  done  by  Fechner  and  Helmholtz, 
how  this  relief  is  accomplished.  The  first  point  will  not  be 
discussed  here.  The  explanation  may  be  summarized  as  fol- 
lows: eye-movement  restores  the  fatigued  retina  by  influencing 
the  metabolic  changes  that  take  place  in  the  fatigued  area;  it 
both  facilitates  the  removal  of  the  fatigue  products  from  this 
area,  and  augments  the  delivery  of  new  nutrient  material  to  it. 

Replying  to  Fick  and  Giirber,  Hering  denies  that  eye-move- 
ment affects  the  visual  processes,  and  explains  the  absence  of 
progressive  fatigue  by  his  conception  of  assimilative  and  dis- 
similative  processes  which  are  mutually  corrective.  He  bases 
his  denial  that  eye-movement  is  a  factor  in  restoring  the  stimu- 
lated retina  to  its  normal  condition  upon  four  experimental 
proofs,  all  adduced  to  show  that  it  neither  causes  the  negative 
after-image  to  disappear  nor  produces  any  other  noticeable  dis- 
turbance in  its  temporal  course.  These  four  proofs  are  as  fol- 
lows, (i)  After  continued  fixation  of  a  stimulus,  the  after- 
image does  not  disappear  when  the  eyes  are  rapidly  moved 
away  from  the  stimulus  and  back  again.  (2)  Movement  of 
the  background  causes  the  after-image  to  disappear;  hence  eye- 
movement  can  possess  no  peculiar  power  to  produce  its  dis- 
appearance. (3)  Near-lying  after-images  due  to  successive 
stimulations  do  not  fluctuate  together.  (4)  Eye-movement 
does  not  cause  the  after-image  to  disappear,  when  it  is  observed 
in  a  darkened  field  of  vision.  These  points  will  be  taken  up 
in  the  order  given. 

(i)  The  demonstration  is  as  follows.     If  a  disc  or  square  of 


INTERMITTENCE  OF   MINIMA!,  VISUAL  SENSATIONS        67 

dead  black  paper  is  laid  on  a  white  background,  and  its  centre 
fixated  for  some  time;  and  if  the  observer  then  moves  his 
eyes  quickly  out  to  some  near-lying  point  and  back  again  to 
the  neighborhood  of  the  stimulus;  the  after-image  is  not  found 
to  have  disappeared  as  a  result  of  the  movement. 

Hering  does  not  draw  any  specific  conclusions  from  this  ex- 
periment alone.  It  will  be  remembered,  however,  that  by 
means  of  it  and  of  the  succeeding  experiments  he  wishes  to 
establish  the  thesis  that  eye- movement  does  not  cause  the  dis- 
appearance of  after-images,  or  otherwise  noticeably  interfere 
with  their  temporal  course,  and  that  it  does  not  factor  in  the 
restoration  of  the  fatigued  retina.  It  seems  fair  to  add,  as 
an  obvious  corollary  to  this  thesis,  that  it  does  not  cause 
the  fluctuation  of  the  negative  after-image.  Now  it  is  evi- 
dent that  the  experiment  is  of  little  or  no  value  in  the  pres- 
ent connection.  («)  For  to  conclude  from  it  that  eye-move- 
ment does  not  cause  after-images  to  disappear  would  be  to 
generalize  from  a  very  special  case,  namely,  from  an  after- 
image of  high  intensity.  The  result  is  very  different  when 
the  after-image  is  weaker;  eye-movement  readily  brings  less 
intensive  after-images  to  disappearance.  In  general,  after- 
images obtained  with  so  long  or  even  with  a  less  long  period  of 
stimulation  must  dim  to  some  extent  (the  amount  depending 
upon  the  time  of  stimulation)  before  eye-movement  can  cause 
them  to  disappear,  (b)  To  conclude  from  it  that  eye- move- 
ment does  not  factor  in  the  restoration  of  the  fatigued  retina  to 
its  normal  condition  would  be  to  apply  a  test  that  is  over- 
strict.  It  is  not  necessary  that  the  after-image  disappear. 
A  dimming  of  the  after-image  should  indicate  partial  restora- 
tion of  the  fatigued  retina.  In  fact,  the  writer  has  shown  in 
the  rough,  by  a  series  of  experiments  to  be  described  in  a  later 
paper,  that  the  restoration  of  the  adapted  retina  is  proportional 
to  the  loss  of  intensity  in  the  after-image.  The  disappearance 
of  the  after-image  corresponds  to  complete  restoration  of  the 
adapted  retina,  and  should  not  be  required  as  evidence  that 
partial  restoration  has  taken  place.  To  demonstrate,  then, 
that  eye-movement  factors  in  the  restoration  of  the  retina,  it 
need  be  shown  only  that  the  after-image  has  lost  in  intensity; 
and  proof  of  this  is  easy,  however  strong  the  stimulation. 
Observations  made  with  reasonable  care  give  the  uniform  re- 
sult that  after-images,  of  whatever  intensity,  are  dimmed  by 
eye-movement.  (c)  To  conclude  from  it  that  eye-movement 
is  not  a  causal  factor  in  fluctuation  would  be  to  ignore  certain 
relevant  facts.  After-images  of  such  intensity  do  not  fluctuate. 
Just  as  they  must  dim,  to  some  degree,  before  voluntary  eye- 
movement  can  cause  their  disappearance,  so  must  they  dim 
before  fluctuation  begins.  If,  therefore,  the  argument  from 


68  FERREE 

analogy  is  to  be  used  at  all,  the  investigator  must  first  deter- 
mine at  what  intensity  after-images  begin  naturally  to  fluctu- 
ate, and  at  what  intensity  voluntary  eye-movement  of  a  suitable 
range  begins  to  cause  disappearance;  and  may  then  ask  whether 
a  rough  correspondence  obtains  between  the  two  points.  This 
procedure  was  followed  by  the  writer  with  a  number  of  obser- 
vers; and  the  results  show,  uniformly,  an  exceedingly  close  cor- 
respondence. A  description  of  the  method  used  and  a  statement 
of  the  results  are  given  further  on,  p.  1035.  Obviously,  then, 
nothing  can  be  derived  from  this  experiment  that  will  aid  in 
demonstrating  either  the  Hering  thesis  or  its  corollary. 

(2)  In  his  second  observation,  Hering  is  concerned  to  dis- 
prove Fick  and  Giirber's  theory  that  eye- movement  facilitates 
metabolic  change  in  the  retina.  Even  if  disappearance  does 
follow  movement  of  the  eyes,  he  says,  it  is  not  necessarily  im- 
plied that  eye-movement  possesses  any  peculiar  power  to  cause 
disappearance;  for  movement  of  the  background  yields  the 
same  result.  The  effect  of  moving  the  background  is  ex- 
plained as  follows :  "Dies  hat  seinen  Grund  in  der  Wechsel- 
wirkung  der  Sehfeldstellen  und  zuin  Theile  auch  darin,  dass 
die  Augenmedien  nicht  ganz  homogen  sind  und  daher  immer 
mehr  oder  weniger  Licht  von  der  Bahn  abirrt,  die  wir  ihm  the- 
oretisch  zuschreiben."  The  explanation  would  seem  to  give 
up  the  whole  controversy;  for,  as  it  stands,  precisely  the  same 
effect  should  be  produced  by  moving  the  eyes  as  by  moving 
the  background.  However,  we  let  this  point  pass,  and  pro- 
ceed to  consider  the  statement  that  movement  of  the  back- 
ground causes  disappearance.  That  is  true.  It  is  possible, 
within  limits,  to  duplicate,  by  movement  of  the  background, 
any  fluctuation  series  that  may  be  produced  by  voluntary  eye- 
movement.  In  all  such  cases,  however,  the  eye  is  tempted  to 
movement  by  the  moving  background.  At  any  rate,  when  the 
eye  is  held  steady,  movement  of  the  background  does  not 
cause  the  after-image  to  disappear.  This  may  be  demonstrated 
as  follows,  (a)  Use  for  the  background  the  mottled  surface 
presented  by  the  darker  side  of  engine-gray  cardboard.  Pro- 
ject the  after-image.  Let  it  become  sufficiently  dim,  and  move 
the  cardboard  in  any  direction.  Disappearance  takes  place. 
Now,  place  a  fixation  point  immediately  in  front  of  the  back- 
ground; e.  g.,  a  black  knot  in  a  taut  vertical  white  cord.  Fix- 
ate this  steadily.  Movement  of  the  background  scarcely  dims 
the  after-image.  (£)  The  following  method  is  probably  not 
so  fair  a  test  of  Hering' s  position  as  that  just  described,  since 
he  asserts  that  a  change  in  the  illumination  of  any  part  of  the 
retina  acts  reciprocally  on  other  parts.  Hence  the  maximal  ef- 
fect would  be  produced,  we  may  suppose,  by  movement  of  the 
whole  background,  and  not  by  movement  of  the  particular  area 


INTERMITTKNCE   OF   MINIMAL  VISUAL  SENSATIONS        69 

upon  which  the  after-image  is  projected.  However,  the  facts 
may  speak  for  themselves.  Use  the  same  mottled  engine-gray 
cardboard  for  the  background.  Place  just  in  front  of  it  a  sheet 
of  same  kind  of  cardboard,  with  a  hole  of  the  exact  size  and 
shape  of  the  after-image  to  be  observed.  Looking  through  this 
hole,  project  the  after-image  upon  the  background.  Move 
this  in  any  direction.  Now  that  the  major  portion  of  the  field 
of  vision  is  steady,  the  shifting  of  the  area  upon  which  the 
after-image  is  projected  does  not  noticeably  disturb  fixation, 
and  correspondingly  does  not  cause  the  after-image  to  disap- 
pear. (Instead  of  a  sheet  of  cardboard,  a  disc  mounted  upon 
a  color- mixer  may  be  placed  behind  the  opening.  When  the 
after-image  is  projected  upon  it,  the  disc  may  be  rotated  at  any 
chosen  rate  of  speed  without  sensibly  dimming  the  image. ) 

(3)  Hering's  third  demonstration  is  as  follows.  Place  a 
short,  broad  strip  of  colored  or  dark  paper  on  a  white  back- 
ground 5  mm.  to  the  left  of  a  fixation  point.  Observe  for  10 
sec.  Quickly  remove  it  and  place  a  similar  strip,  parallel  to 
the  position  of  the  first,  5  mm.  to  the  right  of  the  fixation 
point.  Observe  for  10  sec.  Remove  this,  and  replace  the  first 
strip  for  10  sec.  Then  remove  the  first,  and  replace  the  second 
strip  for  10  sec.  Thus  the  eyes  have  been  exposed  to  both 
strips  for  20  sec. ,  with  an  intermission  for  each  of  10  sec.  The 
object  of  this  arrangement  was,  apparently,  twofold :  first,  by 
successive  stimulation,  to  start  the  after-images  in  different 
phases  of  oscillation,  and  thus  to  cause  them  to  fluctuate  suc- 
cessively; and  secondly,  by  causing  them  to  fluctuate  succes- 
sively, to  show  that  eye-movement  cannot  have  been  responsi- 
ble for  their  fluctuation,  but  rather  that  oscillation  is  grounded 
in  the  nature  of  the  visual  processes.  With  regard  to  the  first 
point,  however,  it  can  be  shown  that  there  is  no  especial  virtue 
in  successive  stimulation  to  produce  successive  fluctuation  in 
after-images  situated  on  different  parts  of  the  retina.  If  two 
stimuli,  not  too  large,  are  placed  at  a  certain  distance  apart  and 
allowed  to  act  simultaneously,  their  after-images  rarely  fluctu- 
ate together.  This  is  one  of  the  common  phenomena  of  fluc- 
tuation, whatever  the  temporal  character  of  the  stimulation, 
and  it  is  in  nowise  essentially  dependent  upon  successive  stim- 
ulations. With  regard  to  the  second  point, — that  if  eye- move- 
ment had  anything  to  do  with  the  fluctuations,  the  after-images 
should  have  fluctuated  together,  and  not  independently  of  each 
other, — we  urge  that  the  conclusion  does  not  by  any  means  follow 
from  the  premisses.  It  is  true,  as  is  pointed  out  by  Hering, 
that  both  areas  of  the  retina  had  been  stimulated  for  the  same 
length  of  time;  and  that,  so  far,  the  after-images  should  have 
been  affected  alike  by  eye-movement.  But  Hering  overlooks 
the  fact  that  the  one  after-image  had  been  fading  for  10  sec. 


70  FERREE 

before  the  stimulus  to  the  other  was  removed.  It  had  thus  run 
a  large  part  of  its  course  before  the  other  began,  and  hence 
might  be  expected  to  disappear  under  a  range  of  movement 
that  would  scarcely  dim  the  other.  We  cannot  only  say  that 
eye-movement  may  have  been  the  cause  of  the  independent 
fluctuation,  but  we  can  go  farther  and  say  that  the  after-images 
behaved  precisely  as  they  should  have  done  if  eye-movement 
were  the  cause  of  their  fluctuation. 

But  farther,  Hering's  result,  as  well  as  his  conclusion,  must 
be  called  in  question.  The  writer  has  tried  the  experiment 
upon  himself  and  a  number  of  observers,  and  so  far  from  find- 
ing Hering's  results  invariable  or  even  typical,  has  rarely  met 
with  a  case  in  which,  after  the  first  couple  of  fluctuations,  the 
one  strip  disappeared  as  a  whole  while  the  other  remained  in- 
tact. Instead  of  that,  the  whole  area  formed  by  the  two  broad 
strips  and  the  narrow  contrast  strip  between  fluctuated,  either 
as  a  whole  or  in  parts,  as  all  after-images  of  a  certain  magni- 
tude do.  When  the  fluctuation  was  in  parts,  now  a  corner  of 
the  area  would  drop  away,  now  a  strip  across  the  top,  now  an 
irregular  patch  here  followed  by  an  irregular  patch  there,  etc. , 
etc.  When  the  area  fluctuated  as  a  whole,  first  the  two  outside 
strips  would  spread  over  the  intermediate  space,  the  whole  area 
becoming  dim  in  consequence;  then  the  entire  image  would 
disappear.  When  observation  was  made  on  the  closed  lids, 
this  experiment  furnished  an  excellent  demonstration  of  the 
relation  of  the  'streaming  phenomenon'  to  fluctuation.1 

Hering's  arrangement  failed  to  give  successive  fluctuation 
for  the  reason  that  the  strips  were  too  large  for  their  distance 
apart.  In  this  zone  of  the  retina,  the  zone  of  fluctuation  in 
parts,  the  area  included  in  each  disappearance  was  not  large 
enough  to  include  the  whole  strip.  Had  the  strips  been  placed 
farther  apart,  or  made  smaller  and  placed  as  Hering  directed, 
the  successive  fluctuations  aimed  at  would  have  been  uni- 
formly obtained;  but  their  occurrence  it  is  plain,  would  in  no 
wise  have  demonstrated  the  intrinsically  oscillatory  character  of 
the  underlying  visual  processes.  Still  better  results,  however, 
would  have  been  obtained  if  the  stimuli  had  been  smaller  and 
also  placed  farther  apart.  A  square  or  rectangular  after-image, 
large  enough  to  include  the  strip-areas,  would  also  have  fluctu- 
ated successively  in  parts,  the  fluctuating  area  now  and  then 
corresponding  to,  or  including,  the  two  strip-areas,  in  turn. 
All  these  fluctuation  phenomena  are  due  to  variation  in  the 

1  For  a  description  of  this  phenomenon,  see  below,  p.  114,  and  for  an 
explanation  of  its  relation  to  the  after-image,  p.  123.  The  stream 
could  be  plainly  seen  to  diffuse  the  color  or  gray  of  the  two  outside 
strips  over  the  intermediate  strip,  and  finally  to  blot  out  the  whole 
image. 


INTERMITTKNCE   OF   MINIMAL  VISUAL  SENSATIONS        71 

area  of  the  retina  involved,  and  have  nothing  to  do  with  suc- 
cessive stimulation.  Over  a  certain  range  of  areas,  fluctuation 
in  parts  is  the  invariable  occurrence,  whatever  the  temporal 
character  of  the  stimulation. 

We  may  now  sum  up  the  discussion  of  this  experiment. 
So  far  from  showing  by  his  special  device  of  successive  stimula- 
tions that  eye-movement  cannot  be  the  cause  of  fluctuation, 
and  so  far  from  throwing  any  difficulty  in  the  way  of  the  eye- 
movement  hypotheses,  Hering  has  succeeded  rather  in  making 
it  easier  to  explain,  by  eye-movement,  the  results  which  he  ob- 
tained. In  other  words,  he  has  produced  the  phenomenon  of 
fluctuation  in  parts  in  the  only  way,  known  to  the  writer,  in 
which  it  admits  of  ready  explanation  by  the  theories  of  Fech- 
ner,  Helmholtz,  and  Kick  and  Giirber.  In  terms  of  any  eye- 
movement  hypothesis,  the  difference  in  intensity  of  the  two 
after-images  is  amply  sufficient  to  explain  why  a  given  eye- 
movement  should  not  affect  both  of  them  alike.  Bering's  re- 
sults woulcl  have  been  more  difficult  to  explain  had  he  used 
simultaneous  stimulation.  It  is  much  more  difficult,  e.  g.^ 
to  say  in  terms  of  eye-movement  why  after-images  of  a  cer- 
tain area  fluctuate  in  parts,  or  why  small  after-images  due 
to  simultaneous  stimulation  of  different  parts  of  the  retina 
fluctuate  independently  of  one  another.  The  fluctuation  in 
the  case  both  of  simultaneous  and  of  successive  stimulation  is, 
however,  of  the  same  nature,  and  its  cause  must,  evidently,  be 
assigned  upon  other  grounds  than  those  here  given  by  Heriug. 

(4)  As  a  final  step  in  his  argument,  Hering  maintains  that, 
in  order  to  a  final  decision  of  the  question  whether  eye-move- 
ment exerts  any  influence  upon  the  disappearance  of  the  after- 
image other  than  by  causing  changes  in  the  illumination  of 
the  retina,  the  course  of  the  after-image  must  be  traced  in  a 
darkened  field  of  vision.  He  maintains  that  under  these  con- 
ditions eye-movement  does  not  noticeably  alter  the  natural 
course  of  the  after-image,  much  less  cause  it  to  disappear.  A 
few  sentences  further  on,  however,  he  qualifies  this  statement 
by  the  remark  that,  when  observing  in  a  dark-room,  he  was 
never  able  entirely  to  blot  out  an  after-image  that  was  at  all 
distinct  or  intensive,  by  moving  the  eyes.1 

1  "In  den  ersten  Paragraphen  meiner  Mittheilungen  zur  Lehre  vom 
1/ichtsinne  habe  ich  eine  Reihe  von  Erscheinungen  besprochen,  welche 
man  an  Nachbildern  im  geschlossenen  und  verdunkelten  Auge  be- 
obachtet.  Ich  hatte  bei  solchen  Versuchen  reiche  Gelegenheit  fest- 
zustellen,  dass  Augenbewegungen  den  gesetzmassigen  Verlauf  dieser 
Nachbilder  gar  nicht  merklich  beeinflussen.  Auch  habe  ich  zahl- 
reiche  Versuche  in  einem  Zimmer  angestellt,  welches  vollstandig  ver- 
dunkelt  werden  konnte,  nachdem  ich  mir  das  Nachbild  erzeugt  hatte. 
Hier  hatte  ich  den  Vortheil,  die  Augenbewegungen  bei  ebenfalls 
offenen  Augen  ausfiihren  zu  konnen.  Nie  war  es  moglich  ein  irgend 


72  FKRRKE 

Apparently,  there  is  here  a  tacit  admission  that  eye-move- 
ment causes  weaker  or  less  distinct  after-images  to  disappear. 
If  so,  the  argument  against  it  as  a  causal  factor  is  materially 
weakened ;  for,  as  has  been  stated  earlier  in  the  discussion, 
intensive  after-images  do  not  fluctuate  at  all.  They  must  be- 
come sufficiently  dim  if  fluctuation  is  to  set  in.  In  any  event, 
however,  observation  is  the  court  of  final  appeal.  We  must 
determine,  first,  whether  eye-movement  does  cause  after-images, 
weak  or  strong,  to  disappear  when  they  are  observed  in  a 
darkened  field  of  vision;  and  secondly,  whether  the  point  at 
which  fluctuation  begins  roughly  coincides  with  the  point  at 
which  eye-movement  first  causes  the  after-image  to  disappear. 
A  series  of  experiments  was  conducted  to  this  end.  Many 
stimuli  were  used,  colored  and  gray,  and  the  after-images  were 
observed  under  the  following  conditions  :  with  the  eyes  closed 
and  carefully  covered  by  a  black  cloth;  in  the  dark-room,  with 
the  eyes  both  open  and  closed;  and  in  the  blackness  cylinder. 
In  every  case  eye-movement  caused  disappearance  when  the  af- 
ter-image had  become  sufficiently  dim,  and  this  point  roughly 
corresponded  with  the  point  at  which  fluctuation  began.  The 
following  results,  which  are  typical,  have  been  selected  for 
publication.  In  this  case  the  stimulus  was  a  square  of  Hering 
white  paper,  5  by  5  cm.,  on  a  background  of  Hering  gray  no. 
3 1 ;  and  the  after-image  was  observed  with  the  eyes  closed  and 
covered  with  a  black  cloth.  Miss  Alden,  a  graduate  student 
in  psychology  in  the  University  of  Colorado,  acted  as  observer. 
The  time  of  stimulation  was  40  sec. ,  and  the  distance  of  the 
observer  from  the  stimulus  was  75  cm.  The  recording  appa- 
ratus consisted  of  kymograph,  telegraph  key  and  electro-mag- 
netic recorder,  and  electro- magnetic  time-marker  in  circuit  with 
a  small  chronometer  set  to  half-seconds.  When  the  disappear- 
ances were  caused  by  eye-movement,  the  eyes  were  moved 
every  3  sec.,  at  a  signal  from  the  experimenter.  It  may 
be  well  to  add  that  the  results  showing  the  closest  approxima- 
tion have  not  been  chosen  for  publication.  In  the  case 
selected,  too  much  eye-movement  was  prescribed.  The  after- 
image began  to  fluctuate  at  a  greater  intensity  than  in  the 
companion  series  of  natural  fluctuations.  There  were  more 
frequent  fluctuations,  and  the  average  phase  of  visibility  was 
shorter.  Erring  as  they  do,  however,  on  the  side  of  making 
eye-movement  too  effective,  these  results  tell  more  strongly 
against  Bering's  assertion  that  eye- movement  does  not  cause 
disappearance  in  a  darkened  field  of  vision  than  do  the  results 

deutliche  Nachbild  durch  Augenbewegungen,  auch  wetm  sie  un- 
gewohnlich  gross  und  lebhaft  waren,  zum  verschwinden  zu  bringen.'* 
In  v.  Graefe's  Archiv,  XXXVII,  2,  1891,  2$.—Cf.  also  S.  Exner,  Zeits. 
f.  Psychol.,  I,  1890,  47. 


INTERMITTENCB  OF   MINIMAL  VISUAL  SENSATIONS        73 

which  show  a  closer  approximation.  For  this  reason,  and  also 
because  about  the  same  amount  of  eye-movement  was  prescribed 
as  in  the  other  duplication  experiments  which  had  already  been 
carried  out,  this  particular  series  has  been  selected. 

TABLE  I 

A.     Showing  results  for  a  darkened  field  of  vision  when  the  fluctua- 
tions were  natural,  and  when  they  were  produced  by 
voluntary  eye-movement.     Unit  i  sec. 


TYPE  OF 
FLUCTUATION 

No.  OF 
FLUCT'S 

ISt 

VIS. 

AV. 
VIS. 

TOTAL 

Vis. 

Av. 

INV1S. 

TOTAL 
INVIS. 

VIS.    + 

INVIS. 

Natural 

7 

9-9 

6.6 

46.2 

2.2 

15-4 

61.6 

Produced  by     \ 
Voluntary 
Eye-movement  ) 

8 

6.2 

4.8 

38.4 

2-5 

2O.O 

58.4 

With  regard  to  the  natural  fluctuation  of  after-images,  when 
observed  in  a  darkened  field  ot  vision,  Hering  says:  "An  den 
ersterwahnten  Nachbildern  (i.  e.,  those  due  to  a  bright  object 
seen  on  a  dark  ground,  fixated  for  10-30  sec.)  aber  erfordert  es 
sogar  besondere  Aufmerksamkeit  wahrzunehmen,  dass  das 
negative  Nachbild  nach  langerem  Bestehen  nicht  bloss  vorii- 
bergehend  verschwindet,  sondern  dass  sich  zwischen  sein  Ver- 
schwinden  und  eventuelles  Wiedererscheinen  eine  schwache 
positive  Phase  einschiebt,  die  freilich  oft  genug  iiberhaupt  nicht 
merklich  wird."1  It  is  difficult  to  determine  whether  this 
statement  means  that  the  weak  positive  phase  occupies  all  of 
what  usually  passes  for  the  phase  of  disappearance,  or  only  a 
part  of  it.  If  it  occupies  the  whole  time,  there  is  of  course  no 
intermission  in  the  after-image  process.  The  disappearance 
usually  observed  is  merely  an  artifact,  produced  by  observation 
with  the  retina  illuminated.  In  view  of  this  uncertainty  it 
seemed  worth  while  to  repeat  the  experiments.  Hering  says 
that  the  recurrence  of  the  positive  phase  may  be  observed  if 
one  fixates  a  bright  object  on  a  dark  ground  for  10,  20,  or  30 
sec. ,  and  then  watches  the  after-image  in  a  darkened  field  of 
vision.  A  square  of  white  paper  on  a  dark  ground  was  taken 
as  stimulus.  This  gave,  as  negative  after-effect,  a  black  square 
with  a  distinct  contrast  border  of  brilliant  white  on  a  very 
light  gray  ground.  The  black  square  fluctuated  frequently; 
but  there  was  never  left  in  its  place,  nor  did  there  ever  appear 
anything  that  resembled,  a  square  of  white  or  light  gray.  When 
it  disappeared,  however,  some  part  of  the  contrast  border  at 
times  remained  momentarily  visible  and  often  could  be  seen  to 


lOp.  tit.,  18. 


74  FERREH 

reappear  slightly  in  advance  of  the  black  square;1  but  this  phe- 
nomenon could  scarcely  be  mistaken  for  a  weak  phase  of  the 
positive  after-image.  A  small,  irregular  patch  of  slightly 
luminous  haze  is  also  frequently  noticed  about  the  point  of 
regard.2  But  this  occurs  just  as  frequently  during  a  phase  of  in- 
visibility when  the  eye  has  been  exposed  to  a  colored  or  white 
stimulus,  or  in  the  darkened  field  of  vision  when  the  retina 
has  undergone  no  local  stimulation  at  all;  hence  it  cannot  be 
a  positive  after-effect  of  stimulation. 

In  order  to  make  the  test-conditions  still  more  favorable  to 
Hering,  we  substituted  for  the  dark  gray  background  pre- 
scribed by  him  a  light  gray  (Hering  no.  15).  Under  the 
original  conditions,  the  positive  phase  must  have  been  difficult 
to  distinguish,  had  it  occurred.  The  negative  after-effect  now 
obtained  was  a  black  square  upon  a  ground  of  gray  such  that 
not  only  would  the  lighter  positive  phase  have  been  easily  dis- 
tinguishable, if  it  occurred,  but  that  it  would  also  have  been 
considerably  intensified  by  contrast.  Still  the  positive  phase 
could  not  be  detected  during  the  periods  of  invisibility  of  the 
negative  phase. 

When  the  stimulus  is  luminous,  Hering  says  that  the  positive 
phases  are  plainly  present,  alternating  with  the  negative.  As 
before,  two  experiments  were  made  upon  this  point  by  our  ob- 
servers: the  one  with  the  background  light,  but  not  so  bright 
as  the  stimulus  ;  and  the  other  with  the  background  dark. 
In  the  first  experiment,  the  sun's  disc  and  Colorado  daylight 
served  as  stimulus  and  background.  The  bright  background 
intensified  the  darkened  field  of  vision  in  the  after-effect,  and 
thus,  as  far  as  brightness  was  concerned,  favored  by  contrast 
the  observation  of  the  positive  phase  of  the  after-image.  The 
eye  was  stimulated,  probably,  from  i  to  3  sec.  The  observa- 
tions were  made  in  mid  summer,  near  the  middle  of  the  day. 
The  sky  was  cloudless,  and  the  light  very  intensive.  While, 
now,  it  is  difficult  to  decide  what  is  positive,  and  what  nega- 
tive, in  the  color  changes  that  follow  exposure  to  a  brightly 


is,  of  course,  is  merely  an  instance  of  fluctuation  in  parts.  The 
area  that  was  swept  out  by  the  stream,  or  complex  of  streams,  did  not 
at  first  include  this  particular  part  of  the  contrast  border;  but  soon, 
owing  to  the  spread  of  the  area  of  commotion,  the  contrast  border  be- 
came involved  in  the  disappearance.  The  reappearance  of  a  part  of 
the  border  in  advance  of  the  remainder  of  Jthe  after-image  is  a  phe- 
nomenon of  the  reverse  order.  The  border  was  cleared  of  the  stream- 
ing material  before  the  rest  of  the  image. 

2  Close  observation  shows  this  to  be  a  centre  of  activity  of  a  'stream- 
ing' area.  It  also  occurs,  but  less  noticeably,  in  other  parts  of  the 
field  of  vision  more  remote  from  the  point  of  regard.  Such  patches 
are  usually  found  to  be  the  foci  or  places  of  intersection  of  narrow, 
swiftly  moving  streams. 


V, 


INTERMITTENCE   OF   MINIMAL  VISUAL  SENSATIONS        75 

luminous  stimulus,  still  our  observers  reported,  here  as  in  the 
former  experiments,  unquestionable,  well  defined  phases  of  dis- 
appearance. Voluntary  eye-movement  also  produced  disap- 
pearance when  the  after-image  had  reached  the  dimness  at 
which  fluctuation  begins. 

When  exposed  to  the  sun's  disc,  the  eye  was  purposely 
moved  in  order  that  the  after-image  might  be  jagged  and  ir- 
regular. This  was  of  some  advantage  for  observation,  because 
the  limbs  of  the  after-image,  owing  to  their  less  intensive  stim- 
ulation, passed  through  the  color  changes  slightly  in  advance 
of  the  body.  The  after-image  was  observed  with  the  eyes 
closed  and  covered  with  a  black  cloth.  The  following  report 
is  typical : 

There  was  first  a  momentary  lasting  over  of  the  stimulus  in  the 
body  of  the  after-image,  while  the  edges  were  a  deep  red.  The  body 
then  changed  to  a  light  blue.  The  red  border,  in  the  meantime,  had 
been  gradually  extending  inwards,  especially  in  the  limbs  of  the 
image.1  The  edges  of  the  central  blue  patch  next  began  to  change 
to  a  yellow-green.  While  this  change  was  going  on,  the  outer  mar- 
gin of  deep  red  was  encroaching,  more  and  more,  upon  the  centre. 
When  the  central  portion  had  become  almost  entirely  yellow-green, 
the  marginal  red  had  taken  on  a  border  of  deep  blue.2  All  the  cen- 
tral portion  next  became  yellow-green.  The  red  border  encroached 
still  farther  upon  the  centre, J'and  was  in  turn  encroached  upon  by  its 
border  of  dark  blue.  The  centre  then  showed  a  tendency  to  become 
light  blue  again.  Finally,  the  whole  image  became  a  deep  red  with 
a  dark  blue  margin.  This  stage  lasted  for  a  comparatively  long 
time,  and  disappearances  were  frequent.  Next,  the  dark  blue  of  the 
margin  spread  gradually  over  the  entire  image.  There  were  later 
a  few  faint  recoveries  (following  disappearances)  of  a  lighter  blue ; 
then  came  in  order  a  light  red-violet,  a  violet-blue,  and  a  dull  dark 
yellow.  In  these  last  fainter  stages  complete  disappearances  were 
especially  frequent. 

The  fluctuations  observed  in  this  experiment  were  not  dif- 
ferent from  those  occurring  under  the  usual  conditions.  The 

xAt  this  stage,  there  was  a  very  noticeable  effect  of  perspective. 
The  red  seemed  to  be  projected  farther  into  the  background  than  the 
blue,  farther  back  even  than  the  general  field  of  vision,  as  if  it  were 
sunk  or  seared  into  the  field.  The  blue  seemed  a  detached  and  float- 
ing patch,  which  was  now  and  then  swept  away  by  a  stream,  changing 
its  shape  as  it  went,  and  dissolving  in  the  stream  body. 

2These  changes  were  not  all  gradual  or  continuous.  Sometimes  the 
central  patch  would  change  from  light  blue  to  the  next  stage,  yellow- 
green,  and  back  again ;  at  times  it  would  go  from  blue  to  the  deep 
red,  and  back  to  blue  again ;  and  sometimes  it  would  disappear  en- 
tirely, frequently  repeating  in  its  reappearance  all  the  color  stages  in 
their  inverse  order.  Sometimes  it  would  come  back  suddenly  to  the 
color  from  which  it  fluctuated ;  and,  very  occasionally,  the  order 
would  be  irregular.  These  changes  were  always  connected  with  the 
streaming  phenomenon.  Light  streaming,  apparently,  caused  the 
changes  from  color  to  color,  while  heavy  streaming  blotted  out 
the  image  entirely.  Recovery  came  with  the  clearing  away  of  the 
streaming  material. 


76  FKRREE 

after-image  fluctuated  as  a  whole,  and  in  parts,  as  all  after- 
images do.  Now  one  of  the  limbs  would  drop  off ;  now  the 
lower  part,  now  the  upper;  now  the  image  would  disappear 
across  the  centre;  and  again  it  would  disappear  completely. 
In  fact,  this  experiment,  so  far  from  furnishing  evidence  against 
fluctuation,  gives  (owing  to  the  long  duration  of  the  after- 
image) an  unusually  good  demonstration  of  the  various  phe- 
nomena that  characterize  fluctuation. 

Experiments  were  also  made  with  a  dark  background.  Here, 
as  before,  observation  showed  clearly  marked  periods  of  disap- 
pearance. The  intermission  was  absolute.  No  part  of  the  in- 
terval was  occupied  by  anything  that  could  be  identified  with 
a  recurrence  of  the  positive  phase  at  low  intensity.  Further 
details  appear  needless,  as  the  conditions  already  described  were 
more  favorable  than  those  of  the  dark  ground  for  the  observa- 
tion in  question. 

It  is  logically  impossible,  then,  to  conclude  from  the  forego- 
ing experiments,  as  Hering  does,  that  eye-movement  is  ineffec- 
tive in  the  disappearance  of  the  after-image,  or  that  fluctuation 
is  merely  the  alternation  of  negative  with  weak  positive  phases 
of  the  after-image.  It  is  obvious,  rather,  that  eye-movement 
is  able  to  cause  the  diappearance  of  any  after-image  that  will 
fluctuate. — 

d.  Exner  argues  against  the  view  that  intermittence  is 
grounded  in  the  nature  of  the  after-image  process.  It  is  a  well- 
known  fact,  he  says,  that  eye-movement  will  cause  an  after-im- 
age to  disappear.  Nor  does  eye-movement  affect  the  after-image 
process.  The  eye,  moving  in  some  particular  direction,  causes 
the  field  of  vision  to  travel  across  the  retina  in  the  opposite 
direction.  This  moving  field  of  vision,  by  distracting  from  the 
perception  of  the  after-image  which  is  stationary  upon  the  re- 
tina, causes  it  to  drop  out  of  clear  consciousness.  When  the 
eye  comes  to  rest,  the  distraction  is  removed,  and  the  after- 
image reappears.1  Now,  this  explanation  accounts,  at  best, 
only  for  our  inability  to  see  the  after-image  while  the  field  of 

1  Exner  and  Hering  thus  agree,  though  from  different  points  of  view, 
that  there  is  no  particular  virtue  in  eye-movement  to  cause  the  disap- 
pearance of  the  after-image.  Movement  of  the  background  works 
just  as  well.  It  is  worthy  of  notice,  however,  that  while  Hering  uses 
the  statement  as  an  argument  for  the  oscillatory  theory,  Exner  uses  it 
as  an  argument  against.  Exner  thinks  it  evident  that  movement  of 
the  visual  field  distracts  from  the  clear  perception  of  the  after-image 
just  as  it  distracts  from  the  clear  perception  of  objects  actually  in  the 
external  visual  field;  and  explains  the  whole  effect  of  movement  of 
the  background  in  this  way.  It  is  an  easy  step,  then,  to  infer  that 
the  disappearances  produced  by  voluntary  eye-movement  are  to  be 
similarly  explained,  and  to  refer  the  fluctuations  occurring  under  the 
conditions  of  normal  fixation  to  involuntary  eye-movement  rather 
than  to  oscillation  of  visual  processes. 


INTERMITTENCE   OF   MINIM AI,  VISUAI,  SENSATIONS        77 

vision  is  in  motion.  It  does  not  account  for  the  invisibility 
of  the  image  after  the  eye  has  come  to  rest.  There  are,  how- 
ever, two  cases  of  this  inability  to  see  the  after-image  while 
the  field  of  vision  is  in  motion.  In  the  one,  the  after-image 
is  vaguely  seen  throughout,  but  cannot  be  seen  clearly  so  long 
as  the  eye  is  in  rapid  movement.  It  comes  out  at  once 
when  the  motion  lags  or  ceases.  This  corresponds  to  Bxner's 
distraction  phenomenon;  but  it  is  not  what  is  ordinarily  meant 
by  disappearance.  The  other  is  a  case  of  true  disappearance. 
The  after-image  goes  out  absolutely.  It  does  not  reappear 
as  the  motion  lags,  and  is  still  invisible  after  the  eye  has  ac- 
curately regained  its  fixation.  In  the  writer's  experiments 
upon  fluctuations  produced  by  voluntary  eye- movement,  a  dis- 
appearance was  not  recorded  unless  the  after-image  remained 
invisible  after  the  observer  had  accurately  regained  his  fixation. 
Disappearances  of  this  sort  were  evidently  not  due  to  distrac- 
tion, for  distraction  had  ceased  before  the  record  began. 

Although  Exner  thus  seems  to  be  mistaken  in  his  view  of 
the  disappearance  produced  by  eye-movement,  his  theory  will 
be  put  to  experimental  test.  A  direct  corollary  from  it  is  that 
the  effect  of  eye-movement  upon  the  disappearance  of  the 
after-image  bears  an  inverse  relation  to  the  uniformity  of  the 
projection  field.  There  are  three  sets  of  conditions  under 
which  this  relation  should  obtain :  («)  disappearance  under 
the  conditions  of  ordinary  fixation;  (b)  disappearance  produced 
by  voluntary  eye-movement;  and  (c)  disappearance  caused  by 
movement  of  the  background.  It  certainly  does  not  obtain 
under  the  first  conditions.  The  after-image  fluctuates  with 
equal  readiness  when  projected  upon  lettered  surfaces,  upon 
mottled  engine- gray  cardboard,  upon  either  the  dull  or  the 
glazed  surface  of  milk  glass  (than  which  there  is  probably  no 
more  uniform  background) ,  and  upon  the  Hering  gray  papers. 
Nor  does  it  seem  to  make  any  difference  which  of  the  above 
backgrounds  is  used,  when  the  disappearance  is  caused  by  vol- 
untary eye-movement.  The  inverse  relation  does,  however, 
seem  to  hold,  within  limits,  when  the  disappearances  are  caused 
by  movement  of  the  background;  the  mottled  backgrounds 
have,  apparently,  more  effect  than  the  uniform.  Now  it  is 
evident  that  the  mottled  background,  travelling  across  the 
retina,  could  distract  no  more  from  the  perception  of  the  after- 
image in  this  case  than  in  the  other  two.  It  would,  however, 
in  proportion  to  its  irregularity,  distract  from  steady  fixation. 
Thus  the  difference  is  to  be  explained  in  terms  of  increased 
eye-movement;  and  again  the  argument  against  eye-movement, 
upon  more  careful  investigation,  is  converted  into  an  argument 
for  some  sort  of  eye-movement  hypothesis. 

As  the  matter  stands,  then,  with  regard  to  Hering  and  Ex- 


78  FERREE 

ner,  eye-movement  must  still  be  taken  into  account  in  the 
explanation  of  the  fluctuation  of  the  negative  after-image. 

ii.     Demonstration  of  causal  connection  between  eye-movement  and 
fluctuation,  as  against  the  theory  of  intrinsic  oscillation. 

a.  Results  in  general. — In  order  that  the  thread  of  the  ar- 
gument may  not  be  lost  in  the  tables  and  details  that  follow,  a 
brief  general  statement  of  results  is  here  given. 

1 i )  Fluctuation  occurs  only  within  a  limited  range  of  after- 
image areas.     It  is  a  matter  of  common  laboratory  report  that 
fluctuation  does  not  take  place  in  the  after-effect  of  general 
adaptation.     The  after-effect  dies  away  gradually.     There  are 
none  of  the  intermittent  variations  of  intensity  that  characterize 
the  after-effect  of  local  adaptation  to  stimuli  of  certain  areas. 
This  fact  is  brought  out  in  practically  all  the  record-books  kept 
by  members  of  the  junior  training  course  at  Cornell  University. 
Careful  tests  have  also  been  made,  with  the  same  result,  by  the 
help  of  observers  trained  to  work  with  just  noticeable  differ- 
ences, by  whom  even  slight  variations  in  intensity  would  have 
been  noticed. 

We  turn  to  the  after-effect  of  local  adaptation.  Here  we 
find  that  fluctuation  occurs  only  within  a  comparatively  limited 
range  of  after-image  areas,  varying  somewhat  for  the  different 
colors  used,  and  for  different  observers.  Large  after-images  do 
not  fluctuate  at  all;  small  after-images  little,  if  at  all;  after- 
images of  mean  area  alone  fluctuate  readily.  If  a  curve  of  fre- 
quency were  plotted  with  the  areas  laid  off  along  the  ordinate 
and  the  frequency  of  fluctuation  along  the  abcissa,  the  curve 
would  start  close  to  the  abcissa,  rise  gradually  until  a  certain 
area  was  reached,  and  then  bend  down  rather  more  sharply  to 
the  abscissa.  This  result  is,  apparently,  incompatible  with  the 
hypothesis  of  intrinsic  oscillation.  Absence  of  fluctuation  for 
a  single  area  would  tell  strongly  against  that  theory;  and  such 
a  range  of  variation  as  is  expressed  in  the  curve  of  frequency 
would  seem  to  condemn  it  absolutely.  The  shape  of  the  curve 
of  frequency,  together  with  the  fluctuation  in  parts  of  after- 
images of  certain  areas,  is  the  most  difficult  problem  that  the 
fluctuation  of  the  after-image  presents  to  theory.  That  eye- 
movement,  acting  in  co-operation  with  streaming,  offers  a  satis- 
factory explanation  of  all  the  variations  resulting  from  change 
in  area  will  be  shown  in  detail  in  its  proper  place. 

(2)  Whatever  renders  fixation  unsteady  increases   the  fre- 
quency of  fluctuation  and  decreases  the  duration  of  the  after-image. 
The  converse  is  also  true  :    whatever  aids  fixation  decreases 
the  frequency  of  fluctuation  and  increases  the  duration  of  the 
after-image.     Various   methods  were  used  to  disturb  and  to 
control  fixation.     In  every  case  records  of  eye-movement  were 


INTERMITTENCE  OF   MINIMAI,  VISUAL  SENSATIONS        79 

taken,  that  showed  both  the  range  and  frequency  of  the  move- 
ments and  the  total  time  during  which  the  eyes  were  in  motion. 
A  quantitative  comparison  could  thus  be  instituted  between 
these  movements  on  the  one  hand,  and  the  frequency  of  fluctua- 
tion on  the  other.  The  results  show  a  high  degree  of  correla- 
tion. 

(3)  The  form  of  the  stimulus  affects  the  frequency  of  fluctua- 
tion.    Experiments  were  made  with  squares  and  with  narrow 
strips  of  equal  area.     The  latter  showed  a  much  greater  liabil- 
ity to  fluctuation.     This  result  can  hardly  be  explained  on  the 
theory  of  intrinsic  oscillation;  the  oscillatory  character  of  the 
retinal  processes  must  be  sensibly  the  same  within  a  square  as 
within  a  narrow  oblong  area.     There  is,  however,  good  reason 
to  believe  that  eye-movement  differs  in  the  two  cases ;  for  when 
the  strip  is  observed,  the  introspective  reports  of  the  observers 
bear  witness  to  a  strong  conscious  tendency  to  look  towards 
the  ends,  to  see  what  is  happening  there.     The  tendency  to 
increased  movement  with  the  strip-images  is  shown  also  in  the 
eye-movement  records.     Eye-movement,  then,  is  a  factor  in 
the  result.     Another  and,  as  we  shall  see  later,  a  very  impor- 
tant factor  is  the  retinal  distribution  of  the  zones  of  streaming. 

(4)  The  arrangement  of  the  stimulus  with  reference  to  the 
direction  of  greatest  eye-movement  affects  the  frequency  of  fluctua- 
tion and  the  duration  of  the  after-image.     A  strip  after-image, 
placed  with  its  breadth  in  the  plane  of  the  greater  range  and 
frequency  of  eye-movement,  fluctuates  more  frequently  and  has 
a  shorter  duration   than   in   the  inverse   arrangement.     The 
greater  frequency  of  fluctuation  is  due  to  the  action  of  the 
greater  amount  of  eye-movement  upon  the  lesser  dimension  of 
the  after-image.     The  point  may  be  demonstrated  as  follows. 
Let  the  disappearance  be  produced  by  voluntary  eye- movement. 
If  these  movements  are  in  the  horizontal  plane,  disappearance  is 
more  frequent  when  the  breadth  of  the  strip  is  in  the  horizontal 
than  when  it  is  in  the  vertical  plane.     Conversely,  if  the  eye- 
movements  are  in  the  vertical  plane,  disappearance  is  more 
frequent  when  the  breadth  of  the  strip  is  in  the  vertical  than 
when  it  is  in  the  horizontal  plane.     An  explanation  will  be 
given  in  Section  iii,  c.     Were  a  periodicity  grounded  in  the 
nature  of  the  after-image  process,  extraneous  influences,  like 
the  form  and  arrangement  of  the  stimulus,  ought  not  thus  to 
affect  the  frequency  of  fluctuation. 

(5)  The  results  grouped  under   (i),   (j)  and  (4)  can    be 
roughly  duplicated  if  voluntary  eye-movement  is  brought  in  to 
cause  fluctuation.     In  the  experiments  under  this  heading,  the 
same  squares  were  used  as  in  ( i ) ;  the  same  strips  and  squares 
as  in  (3) ;  and  the  same  strips  and  arrangements  with  reference 
to  the  direction  of  greatest  eye-movement  as  in  (4).     The  cor- 


8o  FERRBE 

respondence  in  the  results  of  the  two  series  of  experiments  is 
extremely  high.  Here,  then,  is  a  strong  indication  that  eye- 
movement  is  a  causal  factor  in  fluctuation.  Another  factor, 
however,  is  required  for  the  complete  explanation  of  the  results. 
The  methods  employed  in  (i)  and  (3)  were  especially  devised 
to  vary  the  amount  of  involuntary  eye-movement  from  observa- 
tion to  observation.  Yet  their  results  were  approximated  by 
the  introduction  of  voluntary  eye-movement,  the  amount  of 
which  was  kept  constant  from  one  observation  to  another. 
Obviously,  therefore,  a  second  factor  is  at  work,  which  is 
affected  by  eye-movement,  and  which  in  turn  acts  upon  the 
after-image.  A  more  complete  explanation  is  given  later  in 
terms  of  streaming. 

(6)  An  increase  of  the  time  of  stimulation  increases  the  num- 
ber of  fluctuations  of  the  after-image.     The  time  of  stimulation 
ranged  for  the  different  observers  from  10  to  100  sec.    Increase 
of  the  time  of  stimulation  brought  with  it  an  increase  in  the 
intensity   of  the  after-image,    an   increase  in   eye-movement 
(shown  by  the  records),  and  an  increase  in  the  fluctuation  of 
the  after-image.     It  cannot,  of  course,  be  determined  off-hand 
which  of  the  first  two  variations  is  the  cause  of  the  third. 
From  the  evidence  already  at  hand,  it  seems  probable  that  eye- 
movement  is  responsible  for  the  increase  of  fluctuation.     More- 
over, there  is  no  obvious  reason  why  a  more  intensive  after- 
image, on  the  ground  of  its  intensity  alone,  should  begin  to 
fluctuate  sooner  (at  a  greater  intensity)  or  should  fluctuate 
more  frequentiy  after  it  does  begin, — as  happens  uniformly 
under  the  present  conditions.     The  problem  is,  however,  capa- 
ble of  definite  experimental   analysis.     Increase  of  intensity 
can  be  obtained  by  a  method  which  does  not  cause  an  increase 
of  eye-movement;  namely,  by  increase  of  the  intensity  of  the 
stimulus.     In  this  case  there  is  no  increase  of  the  number  of 
fluctuations.     We  thus  have  new  and  positive  evidence  that 
eye-movement  is  a  causal  factor  in  fluctuation.     A  long  period 
of  fixation  increases  involuntary  eye-movement,  and  this  again 
increases  the  frequency  of  fluctuation,  apparently  in  the  same 
proportion. 

(7)  The  observers  most  sensitive  to  the  methods  used  to  distrib- 
ute fixation  showed  the  widest  range  of  variability  of  fluctuation 
and  duration.     The  observers  ranged  from  very  stable  to  very 
sensitive.     The  tables  show  a  variation  in  results  from  individ- 
ual to  individual,  corresponding  to  the  differences  in  sensitiv- 
ity to  disturbances  of  fixation.     Thus,  B  and  A  were  very  sen- 
sitive, W  much  less  sensitive,  and  M  the  least  sensitive  of  all. 
Correspondingly,  B's  and  A's  results  are  very  different  for  the 
different  variations;  W's  less,  and  M's  still  less  different.  This 
correlation,  it  is  clear,  furnishes  evidence  in  favor  of  eye-move- 


INTERMITTENCE  OF  MINIMAL  VISUAL  SENSATIONS        8 1 

ment  as  clear-cut  and  decisive  as  that  to  be  drawn  from  the 
changes  in  result  produced  by  the  different  methods  in  the  case 
of  a  single  individual. 

(8)  Increase  of  practice  in  fixation  brought  with  it  a  decrease 
in  the  frequency  of  fluctuation  and  an  increase  in  the  duration  of 
the  after-image.  Towards  the  close  of  the  semester's  work  it 
became  clear  from  the  eye- movement  records  that  there  was 
an  increased  ability  to  fixate  on  the  part  of  all  the  observers. 
There  was  also  a  corresponding  decrease  in  the  frequency  of 
fluctuation.  The  pitch  of  the  curve  of  frequency  for  the  method 
of  areas,  e.  g.,  was  lessened.  At  both  ends  of  the  series,  areas 
that  had  fluctuated  earlier  would  not  now  fluctuate  at  all,  and 
areas  that  formerly  fluctuated  readily  now  underwent  fewer 
fluctuations.  The  effect  of  practice  was  especially  marked  in 
the  case  of  M.  There  was  also,  as  the  work  advanced,  a  de- 
creased sensitivity  to  the  methods  used  to  disturb  fixation. 
With  practice  fixation  became  progressively  more  stable.  A 
general  straightening  of  the  frequenc)'  curves,  for  all  the  meth- 
ods employed,  was  the  result. 

b.  General  description  of  method  and  apparatus.     All  methods 
were  ruled  out  as  ineffective  that  did  not  produce  changes  in 
result  markedly  greater  than  the  variations  occurring  from 
time  to  time  without  change  of  experimental  conditions.     Ex- 
periments were  planned  in  series  to  be  finished  at  a  single  sit- 
ting.    Results  obtained  at  different  sittings  were  never  com- 
pared directly,  nor  were  the  results  from  broken  or  interrupted 
series  included  in  the  general  averages.     The  order  of  presenta- 
tion of  the  members  of  a  series  was  changed  from  time  to  time, 
in  order  to  rule  out  time  and  practice  errors.     Care  was  taken 
in  the  selection  of  O's  to  get  a  random  sampling  of  types  both 
as  to  visual  organization  and  as  to  experience.     As  little  as 
possible  was  left  to  the  uncontrolled  introspection  of  the  O's. 
The  analyses  were  provided  for  in  the  experimental  variations, 
and  the  O's  were  asked  only  for  the  simplest  judgments,  and 
were  kept  in  entire  ignorance  of  the  problem  and  plan  of  ex- 
perimentation. 

The  recording  apparatus  used  throughout  consisted  of  kymo- 
graph, telegraph-key  and  electro- magnetic  recorder,  with  electro- 
magnetic time-marker  regulated  by  a  chronometer  set  to  half- 
seconds.  The  experiments  were  conducted  in  a  long  optics 
room  lighted  at  the  one  end  by  two  windows  reaching  from 
near  the  floor  to  the  ceiling.  The  0,  head  in  rest,  was  seated 
between  these  windows,  so  that  the  light  coming  from  either 
side  and  above  fell  upon  the  projection- field  of  engine-gray  card- 
board 75  cm.  in  front.  The  time  of  stimulation,  unless  other- 
wise stated,  was  40  sec.,  and  the  unit  of  record  was  i  sec. 

c.  Results  in  detail.     The  work  was  begun  three  years  ago 

JOURNAL — 6 


82  FERREE 

in  the  Cornell  University  laboratory,  and  continued  in  the 
laboratory  of  the  University  of  Arizona  during  the  year  1905-6; 
in  the  laboratory  of  the  University  of  Colorado  during  the  fall 
semester  of  1906-7;  in  the  Cornell  University  laboratory  during 
the  spring  semester  of  1906-7;  and  finished  in  the  laboratory 
of  Bryn  Mawr  College  during  the  fall  semester  of  1907-8.  The 
results  have  been  verified  both  in  drill  courses  and  in  re- 
search, with  a  wide  range  of  observers  of  diverse  training  and 
experience.  The  present  section  of  the  work  was  done  for  the 
most  part  in  the  laboratory  of  the  University  of  Colorado.  A 
part  of  it  has  been  repeated  in  the  laboratory  of  Bryn  Mawr 
College.  The  following  persons  served  as  observers:  Professor 
J.  H.  Bair  (B);  the  Misses  Alden  (A),  Walter  (W),  Mont- 
gomery (M)  and  Wright  (Wr),  students  in  his  laboratory; 
and  Miss  Stout  (S),  a  student  in  the  laboratory  of  Bryn  Mawr 
College. 

In  the  tables  account  is  taken  of  the  following  points:  the 
number  of  fluctuations,  the  first  phase  of  visibility,  the  average 
of  the  phases  of  visibility,  the  sum  of  the  phases  of  visibility, 
the  average  of  the  phases  of  invisibility,  the  sum  of  the  phases 
of  invisibility,  and  the  sum  of  the  phases  of  visibility  and  in- 
visibility. One  less  than  the  number  of  phases  of  visibility  has 
been  taken  as  the  number  of  fluctuations.  The  last  disappear- 
ance has  not  been  counted  in  estimating  the  number  of  fluctua- 
tions. This  number  is  of  value  to  us  as  a  measure  of  the 
disturbance  in  the  after-image  process.  A  still  better  measure, 
however,  is  the  frequency  of  fluctuation,  expressed  by  the 
average  of  the  phases  of  visibility.  The  length  of  the  first 
phase  of  visibility  is  of  importance  as  indicating  at  what  inten- 
sity the  after-image  begins  to  fluctuate.  In  general,  the 
stronger  the  operation  of  disturbing  factors,  the  greater  should 
be  the  intensity  at  which  the  after-image  begins  to  fluctuate. 
Accordingly,  then,  we  should  expect  an  increase  of  eye- 
movement  to  decrease  the  first  phase  of  visibility,  and  a 
decrease  of  eye-movement  to  increase  it. 

The  conventional  use  of  the  term  duration  has  been  departed 
from  in  this  discussion.  By  duration  is  here  meant  the  sum  of 
the  phases  of  visibility.  This  use  of  the  term  is  in  the  first 
place  strictly  accurate,  so  far  as  we  have  any  immediate  pres- 
entation to  consciousness  of  the  after-image  process;  and,  so 
considered,  it  has,  in  the  second  place,  more  direct  bearing 
upon  the  problem  of  the  restoration  of  the  adapted  stimulus. 

No  especial  significance  is  attributed  to  the  phases  of  invisi- 
bility. They  are  included  in  the  tables  merely  that  a  complete 
account  of  the  temporal  course  of  the  after-image  may  be 
given. 

( i )     Fluctuation  occurs  only  within  a  limited  range  of  areas. 


f      "   OF   THE 

f  uwr 

Xr. 


9 


INTERMITTENCE   OF  MINIMAL  VISUAL  SENSATIONS        83 

This  statement  holds  of  all  the  colors  used :  Hering  standard 
red,  green,  blue  and  yellow.  The  result  varied  somewhat, 
however,  for  the  different  colors.  The  red  after-image  showed 
itself  throughout  to  be  the  most  instable.  It  fluctuated  most 
frequently,  had  the  shortest  duration,  and  was  most  affected 
by  the  various  disturbances.  At  the  other  extreme  was  the 
yellow  after-image.  It  proved  the  most  stable  of  all.  For  this 
reason,  since  space  does  not  permit  of  giving  results  from  all 
the  stimuli,  the  yellow  image  derived  from  Hering  blue  as 
stimulus  has  been  selected  for  the  following  tables ;  it  affords 
the  most  rigid  test  of  the  effect  of  eye-movement  on  the  tem- 
poral course  of  the  after-image  in  general. 

Areas  ranging  from  .5  by  .5  cm.  to  61  by  50  cm.  (the  latter 
being  the  dimensions  of  a  single  sheet  of  Hering  paper)  and 
viewed  at  a  distance  of  75  cm.  were  employed.  A  still  larger 
area  was  required  for  some  observers,  if  no  fluctuation  was  to 
ensue.  This  increase  in  size  was  obtained  by  moving  the  stim- 
ulus nearer  to  the  observer. 

It  will  be  noticed  from  the  tables  that,  with  small  areas, 
little  or  no  fluctuation  occurs.  Then  there  is  increase  up  to  a 
certain  point,  namely,  10-20  cm.  square,  when  decrease  begins. 
Fluctuation  disappears  entirely  in  the  neighborhood  of  60-65 
cm.  square. 

If  we  ask  how  far  eye-movement  is  to  be  regarded  as  a  causal 
factor  in  the  increased  fluctuation  from  small  to  medium  areas, 
and  its  consequent  decrease,  we  find  the  following  evidence. 
Over  the  range  of  areas  showing  increase  of  fluctuation  the 
observers  spoke  of  a  strong  conscious  tendency  to  look  away 
from  the  fixation  point  in  order  to  see  what  was  happening 
towards  the  margins.  This  tendency  constituted  a  distracting 
factor  for  fixation.  With  small  areas,  the  whole  after-image  lay 
within  the  field  of  direct  observation;  consequently  there  was 
nothing  to  distract  fixation.  With  the  next  set  of  areas,  the 
edges  passed  into  the  field  of  indirect  observation,  but  were  still 
noticeable.  Hence  they  disturbed  fixation  in  various  ways. 
In  the  first  place,  they  broke  the  uniformity  of  the  field  of 
vision;  and  in  the  second  place  the  observer  was  instructed  to 
register  only  total  disappearances,  disappearances  over  the 
whole  area.  The  margins  were  not  clearly  visible,  and  so 
tempted  the  eyes  to  a  readjustment  which  would  bring  them 
into  the  field  of  clear  vision,  and  thus  make  observation  easier. 
With  the  third  set  of  areas,  the  margins  had  passed  so  far 
from  the  field  of  direct  observation  as  to  be  of  little  concern  to 
the  observer.  The  field  was  of  an  uniform  color  and  bright- 
ness, and  the  margins  did  not  compel  attention. 

The  eye- movement  records  confirm  these  introspections. 
There  is  increase  of  movement  through  the  range  of  areas 


FERREE 


TABI,B  II 

A.    Fluctuation  occurs  only  within  a  limited  range  of  areas.    Results 

showing  the  effect  of  variation  of  area  on  the  fluctuation 

of  the  after-image. 


Area 

No.  of 
Fluctu- 
ations 

ISt 

Vis. 

Av. 
Vis. 

Total 
Vis. 

Av. 

Invis. 

Total 
Invis. 

Vis.  + 
Invis. 

.5x  .scm. 

0 

18.7 

18.7 

18.7 

0 

0 

18.7 

1.5x1.5  " 

i 

48.6 

24.8 

49.6 

5-5 

5-5 

55-1 

5*5     " 

2 

59-5 

21.6 

64.8 

2.4 

4-9 

69.7 

10  X  10    " 

12 

21-5 

59 

76.7 

2.4 

28.8 

105-5 

20  X  20   " 

7 

23-5 

10.0 

80.0 

3-2 

22.4 

102.4 

40x40  " 

3 

42.0 

16.0 

64.0 

2.1 

6-3 

70.3 

61  x  50  " 

o 

72.0 

72.0 

72.0 

0 

0 

72.0 

TABI,B  III.    (Observer  W.) 


Area 

No.  of 
Fluctu- 
ations 

ISt 

Vis. 

Av. 

Vis. 

Total 
Vis. 

Av. 
Invis. 

Total 
Invis. 

Vis.  -f 
Invis. 

i   x  i    cm. 

2 

14.7 

20.2 

65-5 

1.6 

3-3 

69.8 

5^5      " 

2 

18.0 

27.9 

83.8 

I.O 

2.0 

85.8 

TO.  X  10.    " 

3 

15-5 

21.  1 

85.0 

I.O 

3-1 

88.7 

20  X  20       " 

6 

21.0 

u.6 

81.3 

3-9 

n.8 

93-1 

40X40       " 

5 

8.8 

10.5 

63.2 

4-3 

21.45 

84.6 

61  x  50     " 

i 

35-5 

31-6 

63-3 

2.6 

2.6 

65-9 

61  x  50     "     \ 
55  cm.  distant  / 

i 

60.0 

47-7 

95-5 

I.O 

I.O 

96.5 

61  x  50  cm.     "1 
35  cm.  distant  / 

0 

50.5 

50.5 

50.5 

0 

0 

50.5 

showing  an  increase  of  fluctuations,  and  decrease  where  there 
is  a  decrease  of  fluctuation.  Nevertheless,  the  explanation  is 
not  so  simple  as  this  correlation  implies.  The  effect  of  eye- 
movement  on  fluctuation  is  not  direct.  Eye- movement  affects 
the  after-image  only  through  its  effect  on  the  streaming  phe- 
nomenon; and  the  final  word  of  explanation  must  be  deferred 
until  we  come  to  discuss  that  subject. 
Two  methods  were  used  for  the  investigation  of  eye-move- 


INTERMITTENCE  OF  MINIMAL  VISUAL  SENSATIONS        85 
IV.    (Observer  M.) 


Area 

No.  of 

Fluctu- 
ations 

ISt 

Vis. 

Av. 

Vis. 

Total 
Vis. 

Av. 

In  vis. 

Total 

Invis. 

Vis.  + 
Invis. 

1.5  x  1.5  cm. 

o 

52.0 

52.0 

52.0 

0 

o 

52.0 

5x5     " 

2 

27-3 

18.4 

55-2 

i-3 

2-5 

57-7 

10  X  10      " 

6 

35-5 

7-9 

55-3 

2.5 

7-5 

62.8 

20  X  20      " 

6 

35-0 

8.0 

564 

1-3 

7-8 

62.2 

40  x  40     " 

5 

32.1 

8.8 

53-1 

1.6 

8-3 

61.4 

61  x  50     " 

4 

42.2 

10.5 

52.5 

i-3 

5-3 

57-8 

61x50  "  1 

35  cm.  distant   / 

0 

52.0 

52-0 

52.0 

o 

o 

52.0 

ment.  In  the  first  method,  the  shifting  of  the  after-image 
from  the  stimulus  during  fixation  was  chosen  as  a  measure  of 
the  eye- movements  taking  place.  This  method  had  the  disad- 
vantage that  the  eye-movement  could  not  be  recorded  while 
fluctuation  was  going  on.  However,  by  using  only  experi- 
mental variations  that  produced  marked  changes  in  the  steadi- 
ness of  fixation,  by  alternating  eye-movement  with  fluctuation 
records,  and  by  taking  a  large  number  of  records  for  each 
experimental  device,  this  objection  was  practically  obviated; 
more  especially  as  only  comparative  results  were  desired. 
The  method,  too,  has  the  very  great  advantage  of  sensitivity. 
When,  £.  g. ,  the  observer  is  stationed  i  meter  from  the  stimu- 
lus, a  shift  of  the  after-image  i  mm.  to  either  side  represents 
an  eye-movement  (measured  by  the  chord  of  the  arc)  of 
approximately  .017  mm.1  The  sensitivity  of  this  method  is 
directly  proportional  to  the  distance  of  the  observer  from  the 
stimulus,  and  is  limited  only  by  the  range  of  distinct  vision. 
Under  favorable  conditions  exceedingly  slight  tremors  can  be 
detected.  In  fact,  as  a  gauge  for  small  eye-movements,  the 
method  is  far  more  sensitive  than  the  methods  of  photography 
and  of  mechanical  registration. 

In  the  second  method,  the  after-image  was  projected  without 
a  fixation  point,  and  a  record  was  made  of  the  time  during 
which  it  was  moved  and  of  the  time  during  which  it  was  still. 
This  method  was  somewhat  defective,  because  the  range  of 
movement  (a  very  important  factor  in  the  causing  of  fluctua- 
tion) could  be  indicated  only  roughly,  by  the  introspective 

1Calculatipn  is  made  from  the  average  of  the  first  and  second  princi- 
pal focal  distances  of  the  normal  eye  as  estimated  by  Listing:  see 
Helmholtz,  Physiol.  Optik,  90. 


86  FKRREE 

reports  of  the  observers  as  to  whether  the  after-image  moved 
rapidly  or  slowly.  However,  it  proved  a  valuable  supplement 
to  the  former  method,  since  by  it  the  eye-movements  were 
registered  while  the  fluctuations  were  actually  going  on.  One 
could  thus  tell  at  once  whether  disappearance  came  as  a  direct 
effect  of  eye-movement,  i.  e.,  whether  it  came  while  the  eye 
was  moving  or  immediately  after  it  had  moved;  or  whether  it 
came  in  an  interval  of  rest.  The  effect  of  vigorous,  quick 
movements  could  also  be  compared  with  that  of  weaker  and 
slower  movements. 

Two  forms  of  this  second  method  were  employed.  In  the 
one,  the  eye-movement  and  the  phases  of  appearance  and  dis- 
appearance of  the  after-image  were  both  recorded ;  in  the  other, 
the  eye-movements  alone  were  recorded.  The  first  form  gave 
a  direct  tracing  of  the  effect  of  eye-movement  upon  fluctuation. 
The  recording  was  done  as  follows.  The  after-image  was  pro- 
jected without  a  fixation  point,  and  the  key  was  held  down  as 
long  as  the  after-image  was  in  motion  and  released  when  it 
came  to  rest.  When  the  after-image  disappeared,  the  key  was 
given  two  quick  pressures,  and  then  released  until  the  after- 
image returned,  when  the  record  of  movement  went  on  as 
before  until  the  next  disappearance.  A  certain  complication 
arose  with  this  form  of  the  method.  The  effort  to  record  both 
eye-movement  and  fluctuation  seemed  to  interfere  with  the 
course  of  the  after-image,  so  that  fluctuation  occurred  more 
frequently  than  when  fluctuations  alone  were  recorded.  How- 
ever, the  change  was  merely  a  general  rise  in  the  scale  of  fre- 
quency. The  variations  from  device  to  device  stood  out  just  as 
plainly  as  when  the  alternative  method  was  used.  The  reason 
of  the  more  frequent  fluctuations  is,  probably,  that  the  divided 
attention  necessitated,  or  rather  that  the  rapidly  alternating 
direction  of  attention  resulted  in,  a  less  steady  fixation.  On 
this  account,  the  records  that  were  meant  to  show  simply  the 
variation  in  eye- movement  due  to  the  various  devices  were 
taken  according  to  the  second  method:  eye- movement  alone 
was  recorded.  Only  these  results  will  be  given  here,  since  the 
others  cannot  be  adequately  stated  in  tabular  form, and  space 
forbids  the  separate  publication  of  every  set.  The  statement 
must  suffice  that  causal  connection  between  eye-movement  and 
fluctuation  is  directly  evident  upon  the  inspection  of  the  results 
in  question. 

For  the  investigation  of  the  effect  of  variation  of  area  upon 
eye-movement,  the  second  method  (second  form)  was  used. 
The  after-images  were  projected  without  aid  from  fixation;  and 
the  key  was  held  down  as  long  as  they  were  in  continuous 
motion,  and  released  during  the  intervals  in  which  they  were  at 
rest.  The  following  results  were  obtained. 


INTKRMITTENCE  OF   MINIMA!,  VISUAL  SENSATIONS        87 


TABI.B  V 

A.    Eye-movement  with  variation  in  the  area  of  the  stimulus.    Show- 
ing that  an  increase  in  the  area  of  the  stimulus  first  in- 
creases, then  decreases,  the  involuntary  eye- 
movement  occurring  when  the  after- 
image is  observed. 


Area  of 
Stimulus 

Time 
Observed 

Time 
Moving 

Time 
Still 

Time  Moving. 
Time'  Still 

Rate  of 
Move- 
ment 

•5  x  -scm. 

14.5 

4-5 

IO.O 

0-45 

Slow 

1.5  x  1.5  " 

19-5 

6.4 

I3-I 

0.48 

« 

5x5      " 

10  X  10      " 
20  X  20      " 

28.3 
34-0 
37-o 

10.5 
26.45 
20.5 

17.8 
7-55 
16.5 

0-59 
3.50 
1.24 

i< 

Fairly 
Rapid 
Moderate 

40  x  40    " 

65.2 

24-5 

50.7 

0-59 

Slow 

61X50      " 

58-0 

19.4 

38.6 

0.50 

«< 

TABLE  VI.     (Observer  W.) 


Area  of 
Stimulus 

Time 
Observed 

Time 
Moving 

Time 
Still 

Time  Moving 
Time'still 

Rate  of 
Move- 
ment 

i  x  i  cm. 

52.0 

15-0 

37-0 

0.40 

Slow 

2X2       " 

58.5 

17.1 

41.4 

0.41 

« 

10  X  10    " 
20  X  20    " 
40X40    " 

62.0 
68.0 
87.0 

23-3 

37-5 
46.26 

38.7 
30-4 
40.74 

0.60 
1.23 
I.I3 

« 

Fairly 
Rapid 
« 

61  x  50  " 

96.2 

21.3 

74-9 

0.28 

Slow 

(  2  )  Whatever  renders  fixation  unsteady  increases  the  frequency 
of  fluctuation  and  decreases  the  duration  of  the  after-image.  The 
stimulus  was  a  square  of  standard  Hering  blue,  3  by  3  cm. , 
fastened  upon  a  large  square  of  engine-gray  cardboard.  A 
square  of  the  same  cardboard  was  used  as  a  background  upon 
which  to  trace  the  course  of  the  after-image.  The  effectiveness 
of  the  methods  employed  to  disturb  fixation  was  determined  by 
records  which  showed  the  range  and  frequency  of  the  eye- 
movements  produced,  and  the  total  time  during  which  the  eyes 
were  moving.  The  methods  themselves  were  five  in  number. 

(a)  The  stimulus  was  fixated  at  its  centre,  and  the  after- 
image was  projected  without  a  fixation-point.  (£)  The 
stimulus  was  fixated  at  its  centre,  as  before,  and  the  after-image 


88 


FERREE 


TABLE  VII.     (Observer  M.) 


Area  of 
Stimulus 

Time 
Observed 

Time 
Moving 

Time 

Still 

Time  Moving 
Time  Still 

Rate  of 
Move- 
ment 

1.5x1.5  cm. 

50 

7.6 

41-5 

0.18 

Slow 

5-x  5.      " 

10  X  10      " 
20  X  20      " 

56 
60 

58 

25.0 
41-5 
40.5 

31.0 
16.5 
19.5 

0.80 
2.50 

2.08 

« 

Fairly 
Rapid 
« 

40  x  40     " 

60 

39-8 

20.2 

1.98 

Moderate 

61X50      " 

58 

35-2 

22.8 

1-54 

n 

61  x  50     " 

(35cm.  dis.  ) 

57 

21.4 

35-6 

0.60 

Slow 

was  observed  by  help  of  a  fixation- point.  All  the  observers, 
with  one  exception,  found  the  point  of  service  for  holding  the 
after-image  steady.  The  exception  was  Dr.  Bair,  for  whom 
any  effort  at  muscular  control  resulted  in  involuntary  twitch- 
ings.  In  his  case,  therefore,  the  eye-movement  records  showed 
a  greater  unsteadiness  of  regard,  when  effort  was  made  to 
fixate  the  point,  than  when  the  image  was  traced  upon  the 
blank  surface  of  the  cardboard.  (c)  When  the  after-image 
was  projected  without  a  fixation-point,  it  tended  uniformly  to 
move  off  in  some  particular  direction,  varying  with  the  observer. 
Whatever  be  the  explanation  of  this  phenomenon  (it  may  possi- 
bly be  due  to  a  faulty  centering  of  the  image  upon  the  retina, 
itself  the  result  of  some  maladjustment  of  the  visual  mechanism, 
— movement  resulting  as  a  reflex  tendency  to  more  accurate 
fixation) ,  advantage  may  be  taken  of  it  to  exaggerate  or  to 
correct  eye-movement.  In  order  to  exaggerate  the  movement, 
the  direction  of  the  tendency  was  carefully  determined  at  the 
beginning  of  the  experimental  series.  The  stimulus  was  then 
fixated  at  a  point  placed  in  the  line  determined  by  this  ten- 
dency to  movement,  but  in  the  opposite  direction  from  the 
centre  of  the  stimulus.  Part  or  all  of  the  stimulus  was  thus 
thrown  into  indirect  vision,  and  the  tendency  of  the  after- 
image to  move  was  increased, — seemingly  in  proportion  of  its 
displacement  from  the  central  portion  of  the  field  of  vision. 
The  reflex  movement  which  tends  to  centre  the  after-image  in 
the  field  of  vision  added  to  the  natural  tendency  to  movement; 
and  the  after-image,  projected  without  a  fixation  point,  moved 
off  rapidly  in  the  direction  planned.  (d)  To  correct  the 
tendency  to  movement,  the  stimulus  was  fixated  at  a  point 
placed  in  the  line  of  the  movement,  but  in  the  same  direction 
from  the  centre  of  the  stimulus.  The  consequent  displace- 


INTERMITTENCE  OF   MINIMAL   VISUAL  SENSATIONS        89 

ment  of  the  after-image  set  up  a  tendency  to  movement  which 
counteracted  the  natural  tendency.  By  careful  adjustment 
it  was  possible  to  obtain  a  fair  balance  of  the  two  factors, 
so  that  the  after-image  was  held  steady  when  projected  with- 
out a  fixation  point.  Even  under  the  rough  conditions  of 
our  experiments  this  adjustment  proved,  for  some  observers, 
the  best  method  of  controlling  fixation  that  we  could  de- 
vise.1 (e).  For  some  observers,  the  best  aid  to  fixation  was 
found  to  be  a  square,  drawn  on  the  cardboard,  of  exactly  the 
same  size  and  shape  as  the  after-image,  with  a  point  placed 
at  its  centre.  When  the  eye  moved,  the  after-image  was  ob- 
served to  slip  from  the  square  frame;  and  the  observer  was  thus 
able  to  correct  the  movement  before  it  had  attained  any  consid- 
erable range.  With  the  combination  of  square  and  central 
point,  the  observer  had  the  double  advantage  of  the  aid  to  fix- 
ation and  the  conscious  check  upon  movement.  With  the 
point  alone,  there  is  little  or  no  conscious  control  of  move- 
ment; for  the  point  has  to  move  so  far  into  the  field  of  indirect 
observation  that  it  is  recognized  as  occupying  a  different  posi- 
tion before  the  control  is  operative  and  the  eye  can  refixate. 
The  distraction  to  fixation  presented  by  the  sides  of  the  square 
was  probably  little,  because  the  figure  was  small  enough  to  be 
included,  practically  as  a  whole,  in  the  field  of  direct  observa- 
tion. At  all  events,  it  did  not  offset  the  advantage  in  the 
cases  of  M,  A,  and  Wr. 

For  A,  the  after-image,  projected  without  aid  to  fixation, 
first  moved  off  slowly  to  the  left,  but  soon  turned  sharply  and 
moved  much  more  quickly  up  and  to  the  right,  the  latter  being 
the  stronger  component  in  the  movement.  All  fluctuations 
occurred  during  the  second  phase.  Hence  the  drift  to  the  left 
was  disregarded  in  the  methods  used  for  correction  and  exag- 
geration. To  exaggerate  the  movement,  the  fixation  point 
was  placed  9  mm.  below  and  12  mm.  to  the  left  of  the  centre 
of  the  stimulus.  To  correct,  it  was  placed  9  mm.  above  and 
12  mm.  to  the  right  of  the  centre. 

For  B,  the  after-image  moved  up  and  to  the  right.  To  ex- 
aggerate this  movement,  the  fixation  point  was  placed  12.5 
mm.  below  the  centre  of  the  stimulus  and  10.5  mm.  to  the  left. 
To  correct,  it  was  placed  8.5  mm.  above  the  centre  and  8.5  mm. 
to  the  right. 

For  W,  the  after-image  tended  to  move  up  and  to  the  right. 

1  The  direction  of  the  tendency  to  movement,  for  the  different  ob- 
servers, will  be  stated  in  the  discussion  of  the  tables.  For  the  students 
passing  through  the  junior  laboratory  course,  it  seems  most  frequently 
upward  or  upward  and  to  the  right.  The  dominant  component 
appears  to  determine  the  observer's  type  as  to  frequency  and  range  of 
movement  in  the  horizontal  and  vertical  planes. 


FERREE 


TABLE  VIII 

A.   Whatever  renders  fixation  unsteady  increases  frequency  of  fluctua- 
tion of  the  after-image  and  decreases  its  duration.      Whatever 
aids  fixation  produces  the  opposite  effect. 


Variation 

No.  of 
Fluctuat's 

ISt 

Vis. 

Av. 

Vis. 

Total 
Vis. 

Av. 
Invis. 

Total 
Invis. 

Vis.-h 
Invis. 

Square 

i 

44.0 

25-0 

50.0 

6.0 

6.0 

56.0 

With  Point 

2 

39-2 

15-8 

47-4 

5-7 

11.4 

58.8 

Without  Point 

3 

22.0 

10.2 

40.8 

3-9 

11.7 

52-5 

Exaggerated 

5 

9-5 

4-0 

24.0 

1.4 

7.0 

31-0 

Corrected 

2 

36.8 

21-5 

64-5 

4-5 

9.0 

73-5 

IX.     (Observer  B.) 


Variation 

No.  of 
Fluctuat's 

ISt 

Vis. 

Av. 
Vis. 

Total 
Vis. 

Av. 

Invis. 

Total 
Invis. 

Vis. 
+ 
Invis. 

With  Point 

10 

17.5 

3-i 

34-1 

1-3 

13.0 

47.1 

Without  Point 

9 

36.0 

6.2 

62.0 

1.2 

10.8 

72.8 

Exaggerated 

14 

26.0 

1^1 

16.5 

I.O 

14.0 

30:5 

Corrected 

8 

49-5 

9.0 

81.0 

i-5 

12.0 

93-0 

TABLE  X.      (Observer  W.) 


Variation 

No.  of 

Fluctuat's 

ISt 

Vis. 

Av. 

Vis. 

Total 

Vis. 

Av. 

Invis. 

Total 
Invis. 

Vis. 

+ 
Invis. 

Without  Point 

4 

13.0 

15-2 

76.0 

.81 

3-2 

79.2 

With  Point 

3 

16.0 

22.4 

89.6 

•55 

1.6 

91  .2 

Exaggerated 

7 

6.0 

8.2 

65-6 

•52 

3-6 

69.2 

Corrected 

2 

20.7 

32.0 

96.0 

•50 

I.O 

97-0 

Accordingly,  to  exaggerate  this  movement,  the  fixation  point 
was  placed  23  mm.  below  and  23  mm.  to  the  left  of  the  centre 
of  the  stimulus.  The  best  correction  of  the  movement  was 
obtained  by  placing  the  point  12  mm.  above  and  6  mm.  to 
the  right  of  the  centre.  W's  records  showed  an  individual  pe- 
culiarity, in  that  the  first  phase  of  visibility  was  relatively 
short,  while  the  last  was  long.  There  is  no  obvious  explana- 
tion. 


INTERMITTBNCB  OF   MINIMAL  VISUAL  SENSATIONS        9! 
TABLE  XI.     (Observer  M.) 


Variation 

No.  of 
Fluctuat's 

ISt 

Vis. 

Av. 
Vis. 

Total 
Vis. 

Av. 

Invis. 

Total 
Invis. 

Vis. 
+ 
Invis. 

Without  Point 

5 

8.7 

7.0 

42.0 

3-7 

18.5 

60.5 

With  Point 

2 

24.0 

24.1 

72-3 

.67 

i-3 

13-6 

Exaggerated 

6 

2.4 

4-3 

30-1 

i.i 

6.6 

36-7 

Corrected 

i 

37-o 

29.7 

59-4 

•9 

0.9 

60.3 

For  M,  the  after-image  moved  up  and  to  the  right.  To  ex- 
aggerate this  movement,  the  fixation  point  was  placed  12  mm. 
below  the  centre  of  the  stimulus  and  12  mm.  to  the  left.  To 
correct,  it  was  placed  7  mm.  above  the  centre  of  the  stimulus 
and  7  mm.  to  the  right. 

For  Wr,  the  after-image  moved  up  and  to  the  right.     To 

TABLE  XII.     (Observer  Wr.) 


Variation 

No.  of 
Fluctuat's 

ISt 

Vis. 

Av. 

Vis. 

Total 
Vis. 

Av. 

Invis. 

Total 
Invis. 

Vis. 
+ 
Invis. 

Without  Point 

3 

81.5 

26.5 

106.2 

2-3 

9.2 

iiS-4 

With  Point 

2 

92.0 

40.3 

120.9 

1.8 

5-4 

126.3 

Exaggerated 

3 

45-o 

17.2 

68.8 

i.S 

6.0 

74.8 

Corrected 

i 

63.5 

45-0 

90.0 

2.5 

2-5 

72.5 

Square 

i 

71.2 

42.0 

84.0 

2.8 

2.8 

86.8 

exaggerate  this  movement,  the  fixation  point  was  placed  11.5 
mm.  below  and  11.5  mm.  to  the  left  of  the  centre  of  the  stimu- 
lus. To  correct,  it  was  placed  10  mm.  above  and  10  mm.  to 

TABLE  XIII 
W.    Showing  the  effects  of  voluntary  control. 


Variation 

No.  of 
Fluctuat's 

ISt 

Vis. 

Av. 

Vis. 

Total 
Vis. 

Av. 

Invis. 

Total 
Invis. 

Vis. 
+ 
Invis. 

Without  Point  \ 
No  Effort  / 

5 

7.0 

10.8 

64.8 

1.4 

7.0 

71.8 

Without  Point  \ 
Effort  / 

3 

16.7 

19.9 

79.6 

i.i 

3-3 

82.9 

92  FERREE 

the  right  of  the  centre  of  the  stimulus.  Wr's  records  showed 
very  long  phases  of  invisibility. 

It  was  found  that,  when  the  after-image  was  projected  with- 
out a  fixation  point,  frequency  of  fluctuation  was  considerably 
increased  if  the  observer  made  no  particular  effort  to  hold  the 
eyes  steady.  The  following  results  illustrate  this  point. 

This  method  showed,  more  plainly  than  any  other,  the  effect 
of  a  variation  in  the  amount  of  eye-movement  upon  the  fre- 
quency of  fluctuation.  For  this  reason,  the  eye-movements 
resulting  from  the  various  devices  employed  were  studied 
with  some  care.  Three  cases  were  made  of  this  determination  : 
specimen  results  will  be  given  from  each  one. 

( i )  In  order  to  compare  the  movements  occurring,  first,  when 
a  point  is  given  for  fixation,  and  secondly  when  there  is  no  such 
aid,  we  had  recourse  to  the  shift  of  the  after-image  from  a  col- 
ored strip.  Strips  of  Hering's  standard  yellow,  5  by  50  mm., 
were  pasted  on  a  background  of  white  cardboard,  with  the 
shorter  dimension  in  the  plane  in  which  the  eye-movement  was 
to  be  investigated.  To  determine  frequency,  it  was  then  neces- 
sary simply  to  record  the  appearance  of  the  after-image  to 
right  or  left,  above  or  below  the  stimulus.  Separate  series 
were  taken  for  each  plane.  For  the  determination  of  range  of 
movement,  narrow  strips  of  paper  of  the  same  brightness  as 
the  background  were  pasted  successively  2,  4,  6,  8  .  .  . 
mm.  from  the  stimulus,  and  only  those  movements  were  re- 
corded that  shifted  the  image  up  to  or  beyond  them.  While 
the  strips  were  inconspicuous,  so  that  the  eye  was  not  drawn 
away  from  the  fixation  point,  it  was  not  difficult  to  observe 
when  the  image  reached  or  passed  them.  The  strips  were  also 
used  when  frequency  alone  was  to  be  determined,  in  order  that 
the  same  experimental  conditions  might  obtain  throughout. 
The  record  was  made  as  follows.  When  the  objects  of  investi- 
gation were  the  frequency  of  eye-movement,  and  the  total 
times  during  which  the  eyes  maintained  and  lapsed  from  their 
proper  fixation,  the  observer  pressed  the  key  as  the  image  ap- 
peared to  either  side  of  the  strip,  and  held  it  down  until  the 
image  was  again  superposed  upon  the  stimulus.  Then  the  key 
was  released,  and  soon.  Since  the  eyes  may  be  said  to  have  been 
in  motion  for  practically  the  whole  period  during  which  the  im- 
age was  not  superposed  upon  the  stimulus,  the  method  gives  a 
record  of  the  total  time  for  which  the  eyes  were  still  and  of  the 
total  time  for  which  they  were  moving.  When,  again,  the 
range  of  movement  was  investigated,  the  key  was  held  down 
only  while  the  image  was  out  as  far  as,  or  beyond,  the  strips 
which  served  as  range  indicators.  These  records,  therefore, 
show  only  the  times  for  which  the  point  of  regard  was  shifted 
a  given  distance  from  the  fixation  point  and  for  which  it  was 
not. 


INTBRMITTBNCB  OF  MINIMAL  VISUAL  SENSATIONS        93 


The  following  tables  also  inform  us  of  the  direction  of  great- 
est eye- movement, — information  which  we  need  under  (4) 
below. 

TABI,S  XIV 

A.    Eye-movement:  Results  showing  the  movement  in  the  horizontal 

and  vertical  planes t  with  and  without  a  fixation  point. 

Time  of  observation,  i  min. 


Arrange- 
ment 

Fixation 

Range  of 
Movement 

No.  of 
Move- 
ments of 
Given 
Range 

A.  Time 
Eye  Mov- 
ing with 
Given 
Range 

B.  Time 
not  Mov- 
ing with 
Given 
Range 

A 
B 

Vertical 

Without 
Point 

Record'd  all 

85 

48.8 

II.  2 

4-35 

With 

Point 

75 

33-2 

26.8 

1.23 

Horizontal 

Without 
Point 

" 

68 

41-25 

18.75 

2.2 

.•>1 

With 
Point 

« 

64 

27-3 

32-7 

0.83 

Vertical 

Without 
Point 

4  mm. 

30 

13-0 

47-o 

0.28 

With 

" 

Point 

11 

25 

5-1 

54-9 

O.O9 

Horizontal 

Without 
Point 

« 

13 

6-55 

53-45 

O.OI 

•• 

With 
Point 

" 

i 

0.4 

59-6 

0.006 

Vertical 

Without 
Point 

6  mm. 

24 

8.6 

51-4 

0.16 

With 

f  ( 

Point 

" 

15 

4-3 

54-7 

0.007 

Horizontal 

Without 
Point 

« 

9 

4-75 

54-25 

0.008 

With 

Point 

0 

0 

120.0 

94 


FERREE 


TABI,B  XV.     (Observer  B;  time  of  obs.,  i  min.) 


Arrange- 
ment 

Fixation 

Range  of 
Movement 

No.  of 
Move- 
ments of 
Given 
Range 

A.    Time 
Eye  Mov- 
ing with 
Given 
Range 

B.    Time 
not  Mov- 
ing with 
Given 
Range) 

A 
B 

Vertical 

Without 
Point 

Record  'd  all 

62 

32-9 

27.1 

1.  21 

tt 

With 
Point 

« 

80 

39-i 

20.9 

1.87 

Horizontal 

Without 
Point 

d 

40 

28.8 

31-2 

0.92 

«( 

With 
Point 

ti 

56 

31-6 

28.4 

I.  II 

Vertical 

Without 
Point 

2  mm. 

16 

13-5 

46-5 

1.29 

« 

With 
Point 

fl 

26  • 

19.6 

40.4 

0.40 

Horizontal 

Without 
Point 

« 

14 

10.4 

49.6 

0.20 

« 

With 
Point 

(( 

20 

I3-I 

46.9 

0.28 

Vertical 

Without 
Point 

4  mm. 

6 

5-2 

54-8 

0.095 

With 

« 

Point 

f  C 

14 

7-4 

52.6 

0.14 

Horizontal 

Without 
Point 

.    «( 

2 

3-i 

56.9 

0.054 

<( 

With 
Point 

« 

10 

5-3 

54-7 

0.96 

Vertical 

Without 
Point 

7  mm. 

o 

0 

60.0 

With 

(t 

Point 

f| 

4 

2-5 

57-5 

0.043 

Horizontal 

Without 
Point 

r< 

o 

o 

0 

With 

ti 

Point 

2 

I 

59-o 

0.017 

INTERMITTBNCB   OF   MINIMAL  VISUAL  SENSATIONS        95 


XVI.     (Observer  W;  time  of  obs.,  I  min.) 


Arrange- 
ment 

Fixation 

Range  of 
Movement 

No.  of 
Move- 
ments of 
given 
Range 

A.    Time 
eye  mov- 
ing with 
given 
Range 

B.    Time 
not  mov- 
ing with 
given 
Range 

A 
B 

Vertical 

Without 
Point 

Record'dall 

55 

40.43 

19-5 

2.07 

" 

With 
Point 

" 

46 

13.67 

46.32 

0.29 

Horizontal 

Without 
Point 

« 

47 

23-4 

36.6 

0.64 

With 

Point 

35 

10.2 

49.8 

0.20 

Vertical 

Without 
Point 

2  mm. 

45 

19.4 

40.5 

0.48 

With 

Point 

M 

32 

9-7 

50-25 

0.19 

Horizontal 

Without 
Point 

" 

39 

14.5 

45-5 

0.31 

« 

With 
Point 

ii 

28 

8.27 

5I-72 

0.16 

Vertical 

Without 
Point 

4  mm. 

21 

12.2 

47-75 

0.25 

With 

Point 

16 

4-22 

57-75 

0.07 

Horizontal 

Without 
Point 

" 

16 

9-7 

50.3 

0.19 

With 

Point 

10 

3-iS 

56.85 

0.05 

Vertical 

Without 
Point 

6  mm. 

5 

1.9 

58.1 

0.0024 

it 

With 
Point 

ii 

3 

1-15 

58-85 

0.0019 

Horizontal 

Without 
Point 

« 

3 

1.05 

58.95 

0.0018 

4  4 

With 

If 

£ 

Point 

o 

o 

6O.O 

96 


FERREE 


TABLE  XVII 


Arrange- 
ment 

Fixation 

Range  of 
Movement 

No.  of 
Move- 
ments of 
Given 
Range 

A.  Time 
eye  mov- 
ing with 
given 
Range 

B.  Time 
not  mov- 
ing with 
given 
Range 

A 
B 

Vertical 

Without 
Point 

Record'd  all 

28 

39-4 

20.6 

1.91 

•' 

With 
Point 

« 

18 

22.8 

37-2 

0.61 

Horizontal 

Without 
Point 

M 

19 

30-6 

29.4 

1.04 

With 

" 

Point 

" 

15 

19-5 

40-5 

0.48 

Vertical 

Without 
Point 

9  mm. 

3 

1-5 

58.5 

0.025 

With 

(  ( 

Point 

2 

0.7 

59-3 

O.OII 

Horizontal 

Without 
Point 

'< 

2 

0.9 

59-1 

0.015 

With 

(  < 

Point 

0 

0 

60.0 

(2)  The  shift  of  the  after-image  from  the  stimulus  was  used 
to  determine  the  eye-movement  for  each  one  of  the  fixation  de- 
vices. The  square  of  Hering  blue  paper,  3.5  by  3.5  cm.,  was 
observed  in  turn  without  a  fixation  point,  with  a  fixation  point 
placed  at  its  centre,  with  a  fixation  point  displaced  from  its 
centre  so  as  to  exaggerate  the  movement,  and  with  a  fixation 
point  so  displaced  as  to  correct  the  movement.  Thus  fre- 
quency and  total  time  of  movement  were  taken  account  of. 
Only  one  table  will  be  given  to  illustrate  these  determinations. 


TABLE  XVIII 

W.    Showing  the  effect  upon  eye-movement  of  the  fixation  devices 
used  in  Table  X.     Time  of  oas.,  i  min. 


Variation 

Time 
Moving 

Time 
Still 

Time  Moving 
Time*  Still 

Without 
Fixation  Point 

27.9 

32.0 

0.87 

With 
Fixation  Point 

15-3 

44-7 

0-34 

Exaggerated 

38.2 

21.8 

1-75 

Corrected 

7-9 

53-i 

0.15 

INTBRMITTBNCE  OF   MINIMAL  VISUAL  SENSATIONS        97 


(3)  The  eye-movement  for  each  one  of  the  fixation  devices 
was  also  determined  by  the  second  method  (second  form). 
The  after-image  was  obtained  as  in  the  after-image  experiments; 
and  the  key  was  held  down  as  long  as  the  image  was  moving, 
and  released  while  it  was  at  rest. 

TABI.K  XIX 

A.  Showing  the  effect  upon  eye-movement  of  the  fixation  devices  used 

in  Table  VIII 


Variation 

Time 
Ob- 
served 

Time 
Mov- 
ing 

Time 
Still 

Time  Moving 
Time*  Still 

Rate  of 
Movement  * 

Without  fixation 
point 

With  fixation  point 

94.9 
118.0 

82.8 
85.1 

12.  1 
32.9 

6.84 
2.58 

Moderate 

Movem't  slow. 
Correction 
jerky 

Exaggerated 

62.2 

58.5 

3«7 

15.81 

Very  rapid 

Corrected 

125.7 

68.9 

56.8 

1.  21 

Very  slow 

TABLE  XX 

B.    Showing  the  effect  upon  eye-movement  of  the  fixation  devices 
used  in  Table  IX 


Variation 

Time 
Observed 

Time 
Mov- 
ing 

Time 
Still 

Time  Moving 
Time  Still 

Rate  of 
Movement 

With  fixation  point 

112.5 

32.0 

80.5 

o«39 

Movem't  slow. 
Correction 

jerky 

Without  fixation 
point 

94.1 

47-3 

46.8 

I.OI 

Moderate 

Exaggerated 

50.0 

46.6 

3-4 

13.70 

Very  rapid 

Corrected 

159.7 

14.7 

145.0 

O.IO 

Very  slow 

(3)  The  form  of  the  stimulus  affects  the  frequency  of  fluctua- 
tion. The  stimulus  was,  as  in  the  former  experiments,  of 
standard  Hering  blue.  When  squares  were  used,  they  were 
made  so  small  that  their  edges  lay  within  the  field  of  direct 
observation;  they  could  thus  exert  no  influence  to  increase 
eye-movement,  and  we  should  expect  a  minimal  disturbance  of 
the  after-image.  The  strips,  on  the  other  hand,  were  made 

1  The  introspections  as  to  rate  of  movement  have  not  been  incor- 
porated in  the  other  sets  of  eye-movement  tables.  In  general,  when 
the  ratio  'time  moving  -f-  time  still'  is  increased,  the  rate  of  movement 
is  also  increased. 

JOURNAL— 7 


98 


FKRREE 


TABW*  XXI 

M.    Showing  the  effect  upon  eye-movement  of  the  fixation  devices  used 

in  Table  XI 


Variation 

Time 
Ob- 
served 

Time 
Mov- 
ing 

Time 
Still 

Time  Moving 
Time  'Still 

Rate  of 
Movement 

Without  fixation 
point 

With  fixation  point 

84-6 
100.4 

35-1 
8-3 

49-5 
92.1 

0.70 
0.09 

Moderate 

Movem't  slow. 
Correction 
jerky 

Exaggerated 

70.2 

50.0 

20.2 

2.47 

Very  rapid 

Corrected 

139.0 

o 

139.0 

Very  slow 

narrow,  so  that,  as  their  areas  were  equal  to  those  of  the 
squares,  their  ends  were  thrown  into  the  field  of  indirect  ob- 
servation, and  the  tendency  was  towards  increased  eye- move- 
ment. Thus  maximal  disturbance  of  fixation  was  obtained  for 
the  given  area,  and  correspondingly  a  maximal  disturbance  of 
the  after-image  was  expected. 

To  illustrate  :  a  strip  ,5x  .5  cm.  had,  as  its  equivalent  area,  a 
square  of  1.5  cm.;  a  strip  .5  x  10  cm.,  a  square  of  2.2  cm.;  a 
strip  .  5  x  20  cm. ,  a  square  of  3.  i  cm. ;  and  a  strip  .  5  x  40  cm. ,  a 
square  of  4.4  cm.  Only  the  squares  of  2.2,  3.1  and  4.4  cm. 
fluctuated  at  all,  while  the  strips  showed  a  rapid  increase  in 
fluctuation  until  .5  by  40  cm.  was  reached,  when  a  slight  de- 
crease occurred.  A  strip  .5  x  20  cm. ,  e.  g. ,  gave  for  A  8  fluctua- 
tions, with  an  average  phase  of  visibility  of  7.3  sec.;  while  its 
equivalent  square  gave  no  fluctuations  at  all(av.  vis.,  71.5  sec.). 
No  record  was  taken  of  fluctuation  in  parts;  only  total  disap- 
pearances were  registered.  Thus  the  actual  disturbance  suf- 
fered by  the  strip-image  was  taken  account  of  only  in  part. 

It  may  be  deduced  from  the  following  tables  that  the  shape 
of  the  curve  of  frequency  obtained  by  increasing  the  length  of 
the  strips  is  somewhat  different  from  that  obtained  by  increas- 
ing the  area  of  the  squares  (Tables  II-IV).  If  a  curve  were 
plotted  by  laying  off  the  lengths  of  the  strips  along  the  abscissa 
and  the  frequency  of  fluctuation  along  the  ordinate,  the  curve 
would  start  on  or  near  the  abscissa,  rise  fairly  steeply  until  a 
length  of  20-40  cm.  was  reached,  and  then  bend  downward 
slightly.  It  would  not  reach  the  abscissa,  since  with  the 
lengths  of  strip  used  fluctuation  did  not  cease  as  it  did  with 
increase  of  area  when  squares  were  used.  The  reason  of  this 
difference  between  the  results  of  the  two  sets  of  experiments 
will  be  given  later,  in  our  discussion  of  the  streaming 
phenomenon. 


INTERMITTBNCE  OF  MINIMAL  VISUAL  SENSATIONS        99 


TABI.E  XXII 

A.    Form  of  stimulus  affects  frequency  of  fluctuation  of  after-image. 

Results  showing  that  fluctuation  is  more  frequent  when 

stimulus  is  in  form  of  strip,  than  when  it  is  in 

form  of  a  square  of  equivalent  area 


Form 

Area 

No.  of 

Fluctu- 
ations 

ISt 

Vis. 

Av. 

Vis. 

Total 
Vis. 

Av. 

Invis. 

Total 
Invis. 

Vis. 

+ 
Invis. 

Strip 

.5x2  cm. 

0 

42.0 

42.0 

0 

42 

o 

4i-5 

Strip 

.5x5  cm. 

2 

26.0 

14-0 

I.O 

43 

2.0 

44-5 

Square 

1.5x1.5001. 

0 

52-0 

52-0 

0 

52 

0 

5i-5 

Strip 
Square 

.5  x  10  cm. 

2.2X2.2Cm. 

6 
o 

35-0 
61.0 

8.0 
61.0 

1.2 

0 

f 
61 

7-3 

0 

63-4 
61.0 

Strip 

.5x20  cm. 

8 

26.5 

7-3 

1.6 

66 

13-4 

79-8 

Square 

3.ix3.icm. 

0 

72.0 

72. 

0 

73 

0 

71-5 

Strip 

.5  x  40  cm. 

7 

26.0 

9-5 

i-3 

76 

9.4 

85.4 

Square 

4.4x4.40111. 

3 

59-5 

19.0 

2.0 

73 

6.0 

79.0 

TABUS  XXIII.      (Observer  W.) 


Form 

Area 

No.  of 

|Fluc- 
uat's 

ISt 

Vis. 

Av. 
Vis. 

Total 
Vis. 

Av. 
Invis. 

Total 
Invis. 

Vis. 
Invis. 

Square 

•  5x  .scm. 

0 

37-2 

37-2 

37-2 

0 

0 

37-2 

Strip 
Square 

.5*    5;; 
1.5x1-5 

2 
I 

14.7 
20.5 

15-7 
33-2 

47-2 
64-5 

2.0 
I.O 

4.0 

I.O 

Is,2 

Strip 
Square 

•  5x  10  " 

2.2X2.2    " 

3 

i 

"•5 
43-o 

13.2 
34-o 

68!o 

i-4 
2-5 

4.2 

2-5 

59-2 
70-5 

Strip 
Square 

.5X20      " 
3-1x3.1    " 

3 

2 

9-7 
29-5 

15-7 
23-15 

62.8 
69-5 

0.9 
i-5 

2-7 

3-o 

65-5 
72.5 

Strip 
Square 

.5X40      " 
4X4         " 

3 

2 

11.5 
23-0 

19.2 
24-5 

77.0 
73-5 

2-9 

1-3 

4-4 
3-9 

81.4 
77-4 

Strip 

.5X61      " 

3 

6.0 

21.8 

87-5 

3-6 

5-5 

93-o 

IOO 


FERREE 
TABLE  XXIV.     (Observer  M.) 


Form 

Area 

No.  of 

Fluct- 
uat's 

ISt 

Vis. 

Av. 
Vis. 

Total 
Vis. 

Av. 
Invis. 

Total 
Invis. 

Vis. 
+ 
Invis. 

Square 

•  5*  -5  cm. 

0 

55-o 

55-o 

55-0 

0 

0 

55-o 

Strip 
Square 

•5*  5      " 
1.5x1.5 

i 

0 

59-7 
66.4 

39-6 
66.4 

79.2 
66.4 

1-5 

0 

1-5 

0 

80.7 
66.4 

Strip 
Square 

•  5x10      " 

2.2X2.2       " 

4 

i 

32-2 
58.5 

12.5 
33-0 

62.3 
66.0 

2.1 
4.0 

8.4 
4.0 

70.7 
70.0 

Strip 
Square 

.5X20         " 
3-IX3.I 

6 

i 

9.2 
97.0 

8.7 
50-5 

61.0 

IOI.O 

3-2 

I.O 

19.2 

I.O 

80.4 

IOI.O 

Strip 
Square 

.5X40         " 
4.4x4.4      ' 

6 

2 

12.5 
93-o 

7-7 
32-5 

54-5 
97-5 

4.4 

7-i 

26.4 

14.2 

80.9 
in.  7 

Strip 

L5X50         " 

4 

20.5 

IO.2 

51-0 

3-9 

15.6 

66.6 

The  second  method  (second  form)  was  here  used  for  investi- 
gating eye-movement.  The  squares  and  strips  were  projected 
on  a  sheet  of  engine-gray  cardboard,  without  a  fixation  point, 
and  the  times  were  recorded  during  which  they  were  moving 
and  at  rest. 


TABLE  XXV 

A.    Eye-movement,  with  variation  in  form  of  stimulus.    Showing 
that  more  involuntary  eye-movement  occurs  during  obseration 
of  after-image  when  stimulus  is  a  strip  than  when  it  is 
a  square  of  equivalent  area. 


Form  of 
Stimulus 

Dimensions  of 
Stimulus 

Time 
Ob- 
served 

Time 
Moving 

Time 
Still 

Time  Moving 
Time  Still 

Strip 
Square 

•  5  xscm. 
1.5  xi.  5  cm. 

24.12 
35-45 

12.02 

9.40 

12.10 
26.05 

0.98 
0.36 

Strip 
Square 

.5  x  10  cm. 
2.2  x  2.2  cm. 

30.00 
46.80 

15-25 
16.35 

14-75 
30.50 

1.03 
o-53 

Strip 
Square 

.5  x  20  cm. 
3.1  x  3.  i  cm. 

36.55 
50.07 

24-53 
19.98 

12.03 
30.72 

2.04 
0.65 

Strip 
Square 

.5  x  40  cm. 
4.4  x  4.4  cm. 

38-85 
55-8o 

19.50 
25-50 

19-35 
30-30 

1.  01 

0.84 

INTBRMITTBNCE  OF   MINIM AI,  VISUAL,  SENSATIONS      IOI 
TABLE  XXVI.     (Observer  M.) 


Form  of 
Stimulus 

Dimensions 
of 
Stimulus 

Time 
Ob- 
served 

Time 
Moving 

Time 

Still 

Time  Mov'g 
Time'still 

Strip 
Square 

-5x  5  cm. 
1.5x1.5    ' 

36.54 
46.90 

14.92 
8.80 

21.62 

38-10 

0.69 
0.23 

Strip 
Square 

.5x10      " 

2.2X2.2      " 

43-25 
49.20 

21.07 
13-30 

22.18 
35-90 

0-95 
0-37 

Strip 
Square 

•  5X20        " 
3-IX3.I      ' 

49.70 
52.30 

31-23 
19.20 

18.47 
33-10 

i& 

Strip 
Square 

.5X40        " 
4.4x4.4      ' 

54.8o 
57-90 

35-50 
24.70 

19.30 
33-20 

1.84 
0.74 

TABLE  XXVII.  (Observer  W.) 


Form  of 
Stimulus 

Dimensions 
of  Stimulus 

Time 
Ob- 
served 

Time 
Moving 

Time 
Still 

Time  Mov'g 
Time'still 

Square 

•  5x  .5  cm. 

63.0 

15-35 

47-65 

0.32 

Strip 
Square 

•5x  -5     " 
1.5x1-5 

64-5 
82.0 

26.20 
21-55 

38.80 
60.45 

0.68 
0-35 

Strip 
Square 

.5x10      " 

2.2X2.2       " 

73-5 
89.0 

33-90 
32.30 

39-6o 
56.70 

0.86 
0-57 

Strip 
Square 

•5X20         " 
3-IX3-I 

82.0 
89-75 

38.90 
33-50 

42.10 
56-25 

0.90 
0.60 

Strip 
Square 

.5X40         " 
4.4x4.4 

82.0 

48.25 
35-40 

49-25 
46.60 

0.98 
0.76 

(4)  The  arrangement  of  the  stimulus  with  reference  to  the 
direction  of  greatest  eye-movement  affects  the  frequency  of  fluctua- 
tion and  the  duration  of  the  after-image.  The  stimuli  for  A 
were  strips  of  Hering  standard  blue,  .5  cm.  wide  and  of  vari- 
ous lengths;  the  stimuli  for  W  were  strips  of  2  cm.  wide.  These 
were  placed  first  in  the  vertical  and  then  in  the  horizontal 
plane,  and  fixated  at  the  centre  for  40  sec.  The  images  were 
observed  on  a  background  of  engine-gray  cardboard,  with  a 
fixation  point.  The  tables  show  more  frequent  fluctuation,  a 
shorter  first  phase  of  visibility,  and  a  shorter  total  visibility, 
when  the  length  of  the  strips  is  in  the  vertical  plane.  Corre- 
spondingly, the  eye- movement  tables  show  a  greater  range  and 
frequency  in  the  direction  of  the  lesser  dimension  of  the  after- 
image (the  horizontal  plane).  The  results  found  with  the 


102 


FERREE 


trained  observers  have  been  paralleled  in  laboratory  practice. 
Those  published  from  this  latter  class  were  obtained  with  Miss 
Stout  (S),  a  student  at  Bryn  Mawr  College. 


XXIX 

A.  The  arrangement  of  stimulus  with  reference  to  direction  of  great- 
est eye-movement  affects  frequency  of  fluctuation  and 
duration  of  after-image. 


Arrangement 
of  Strip 

Dimensions 
of  Strip 

No.  of 
Fluctu- 
ations 

Av. 
Vis. 

Total 
Vis. 

Av. 

Invis. 

Total 
Invis. 

Vis. 

-h 
Invis. 

Vertical 
Horizontal 

•  5  xscm. 
«<       <( 

2 

o 

n.  i 
47-5 

33-2 
47-5 

o-5 
o 

I.O 

o 

34-3 
47-5 

Vertical 
Horizontal 

.5  x  10  cm. 

«        « 

5 

i 

4.0 
24.1 

24.0 
48.2 

0.9 
2.9 

4-5 
2.9 

29-5 
Si-i 

Vertical 
Horizontal 

.5  x  20  cm. 
«          « 

7 

i 

3-8 
29-3 

3°-4 
58.6 

i.i 
2.9 

7-7 
2.9 

61-5 
38-1 

Vertical 
Horizontal 

.5  x  40  cm. 

6 

3 

8.7 
19.1 

60.9 
76.4 

0.8 
0.7 

4.8 

2.1 

65-7 
78.5 

XXX.     (Observer  W.) 


Arrange- 
ment 

Area 

No.  of 
Fluct- 
uat's 

ISt 

Vis. 

Av. 
Vis. 

Total 
Vis. 

Av. 
Invis. 

Total 
Invis. 

Vis. 

Invis. 

Vertical 
Horizontal 

2x5  cm. 
2x5    " 

2 
2 

13.0 
16.5 

16.4 

21.8 

49.2 
65-4 

i*S 

1.6 

3-o 
3-2 

52-2 
68.6 

Vertical 
Horizontal 

2X10  " 
2X10  " 

2 

2 

22.0 

34-5 

17-3 

22.1 

52-0 
66.5 

i-5 
i-4 

3-o 

2.8 

55-o 
69-3 

Vertical 
Horizontal 

2X20  " 
2X20  " 

3 
3 

24-5 
37-o 

16.6 

21.8 

66.5 
87.2 

2.0 

i-3 

6.0 
3-9 

72-5 
91.1 

Vertical 
Horizontal 

2X40  " 
2X40  " 

4 
3 

22.0 
27.6 

16.6 

22.0 

84.0 
88.0 

1.6 
1.4 

6.4 
4.2 

90.4 
92.2 

Vertical 
Horizontal 

2X50  " 
2X50  " 

3 
3 

12.0 
19-5 

I6.7 
20-5 

66.8 
82.0 

0.9 
0.7 

3-6 

2.8 

70.4 
84.8 

The  eye-movement  records  of  A  and  W,  in  the  horizontal  and 
vertical  planes,  are  given  in  Tables  XIV,  XVI.  In  both  cases, 
for  every  point  recorded,  there  was  marked  excess  in  the  hori- 
zontal plane.  Owing  to  lack  of  time  this  determination  was 
not  made  for  S.  The  fact,  however,  that  in  S's  duplication 
series  the  eye-movement  across  the  strip  was  always  more 


INTERMITTENCE  OF   MINIMAL  VISUAL  SENSATIONS      103 


XXXI.     (Observer  S.) 


Arrange- 
ment 
of  Strip 

Dimensions 
of  Strip 

No.  of 
Fluct- 
uat's 

ISt 

Vis. 

Av. 
Vis. 

Total 
Vis. 

Av. 
Invis. 

Total 
Invis. 

Vis. 
Invis. 

Vertical 
Horizontal 

•  5*  5  cm. 
•5*  5     " 

2 

i 

14.2 
32.0 

9.6 

17.4 

28.8 
34-8 

2.6 

4.0 

5-2 
4.0 

34-o 
38-8 

Vertical 
Horizontal 

.5x10    " 
.5x10    ' 

3 

2 

12.6 

28.9 

7-5 
13-6 

30.0 
40.8 

2.1 

3-5 

6-3 
10.5 

36.3 
51-3 

Vertical 
Horizontal 

.5x20     " 
.5x20     " 

8 

4 

10.4 
27.2 

5-6 
13-3 

50.4 
65-5 

i-5 
2.4 

12.0 
9.6 

62.4 
75-i 

Vertical 
Horizontal 

-5x40    " 
•  5x40 

5 

3 

24-5 
30-4 

9-8 
16.4 

58.8 
65.6 

4.0 

4.6 

20.0 
13.8 

78.8 
79-4 

effective  for  fluctuation  than  that  along  it  indicates  that  the 
greater  frequency  of  fluctuation  when  the  strip  was  arranged 
vertically  was  due  to  an  excess  of  eye-movement  in  the  hori- 
zontal plane. 

(5)  The  results  in  (i) ,  (j)  and  (^)  can  be  roughly  duplicated 
by  using  voluntary  eye-movement  to  cause  the  disappearances. 
The  voluntary  eye-movement  was  regulated  throughout  in  the 
following  manner.  The  after-image  was  observed  with  the 
aid  of  a  fixation  point.  A  second  point  was  placed  12  cm.  to 
the  right  of  this.  At  a  signal,  given  every  3  sec.  by  the  ex- 
perimenter, the  observer  moved  his  eyes  quickly  out  to  this 
point  and  back  again.  He  was  told  to  record  as  a  'disap- 
pearance' only  a  case  in  which  the  after-image  failed  to  reappear 
after  the  eyes  had  regained  their  normal  fixation.  Thus  noth- 
ing but  genuine  disappearances  were  taken  account  of.  Possible 
visual  synsesthesia  attending  eye-movement,  distraction,  etc., 
were  guarded  against  by  the  directions  under  which  the 
observer  worked.  The  after-images  were  blotted  out  as  com- 
pletely as  after-images  ever  are  in  the  case  of  natural  fluctua- 
tion. There  is  not  a  shadow  of  doubt  on  this  point.  The 
more  uniform  side  of  engine-gray  cardboard  was  used  as  back- 
ground for  both  stimulus  and  after-image.  Hence  there  was 
no  danger  of  disturbance  by  possible  distractions  due  to  move- 
ment of  the  eye  over  an  irregularly  marked  surface. 

i.  Fluctuation  occurs  only  within  a  limited  range  of  areas. 
Just  as  when  the  observation  is  made  under  the  conditions  of 
ordinary  fixation,  the  after-effect  of  general  adaptation  does 
not  fluctuate  under  the  influence  of  voluntary  eye-movement. 
Nor  do  after-images  fluctuate  beyond  a  comparatively  limited 
range  of  areas.  Within  this  range  the  results  are  very  similar 
to  those  obtained  in  the  case  of  natural  fluctuation.  Large 
images  do  not  fluctuate  at  all;  small  images  little,  if  at  all; 


io4 


FKRREE 


while  middle-sized  images  alone  fluctuate  readily.  The  curve 
of  frequency  takes  the  same  general  shape  as  it  does  with  nat- 
ural fluctuation. 


TABWC  XXXII 

A.    Duplication  of  results  by  voluntary  eye-movement.      Method   of 
variation  of  area. 


Area 

No.  of 
Fluct- 
uat's 

ISt 

Vis. 

Av. 
Vis. 

Total 
Vis. 

Av. 

Invis. 

Total 
Invis. 

Vis. 
+ 
Invis. 

.5x  .5  cm. 

o 

35-5 

35-5 

35-5 

0 

o 

35-5 

1.5x1.5     " 

I 

54-5 

30.0 

60.0 

0.6 

0.6 

60.6 

5^5        " 

3 

59  -° 

18.2 

72.8 

1-5 

4-5 

77-3 

10x10      " 

13 

23.2 

6.5 

91.0 

0.7 

9.1 

IOO.I 

20X20         " 

12 

32.0 

6.2 

80.6 

0.6 

7.2 

87.8 

40x40      " 

2 

78.5 

30.5 

9i-5 

0.9 

1.8 

93-3 

61x50    " 

0 

89.7 

89.7 

89.7 

0 

0 

89.7 

TABI,B  XXXIII.     (Observer  W.) 


Area 

No.  of 
Fluct- 
uat's 

ISt 

Vis. 

Av. 
Vis. 

Total 
Vis. 

Av. 
Invis. 

Total 
Invis. 

Vis. 
Invis. 

.5x  .5  cm. 

o 

33-o 

33-o 

33-o 

0 

0 

33-o 

I  XI         " 

3 

26.2 

10.  0 

40.2 

0-55 

i-7 

41.9 

5x5      " 

7 

"•5 

5-2 

43-3 

o.75 

5-3 

48.6 

10  X  10     " 

14 

19.4 

5-o 

75-9 

0.6 

7-9 

83.8 

20  X  20     " 

13 

20.2 

3-7 

52.0 

1.6 

21.  1 

73-i 

40X40     " 

o 

82.0 

82.0 

82.0 

82.0 

61X50      " 

0 

69.5 

69-5 

69-5 

69.5 

INTBRMITTENCE   OP  MINIMAL  VISUAL  SENSATIONS      105 
TABLE  XXXIV.     (Observer  M.) 


Area 

No.  of 
Fluct- 
uat's 

ISt 

Vis. 

Av. 

Vis. 

Total 
Vis. 

Av. 

Invis. 

Total 
Invis. 

Vis. 

+ 
Invis. 

•  5x  .5  cm. 

o 

47.0 

47.0 

47.0 

0 

o 

47.0 

1.5x1-5     " 

0 

71.0 

71.0 

71.0 

0 

0 

71.0 

10X10         " 

II 

54-5 

6-5 

82.5 

2.0 

22.8 

105-3 

20X20         " 

II 

3i-5 

5-2 

63-1 

i-5 

17.1 

80.2 

4OX4O         " 

10 

37-o 

5-4 

60.0 

i-3 

13-6 

73-6 

61X50         " 

o 

46.0 

46.0 

46.0 

0 

0 

46.0 

it.  The  form  of  the  stimulus  affects  the  frequency  of  fluctua- 
tion. The  same  set  of  stimuli  were  used  as  for  natural  fluctua- 
tion, and  all  the  other  conditions  of  the  experiment  were  kept 
as  nearly  as  possible  the  same.  It  will  be  observed  that  here, 
as  before,  the  squares  fluctuated  little,  if  at  all,  while  the  strips 
increase  in  frequency  of  fluctuation  with  increase  of  length 
until  a  certain  point  is  reached,  when  a  slight  decrease  takes 
place. 


XXXV 

A.    Duplication  of  results  by  voluntary  eye-movement.    Results  show- 
ing that  fluctuation  is  more  frequent  when  stimulus  is  a  strip 
than  when  it  is  a  square  of  equivalent  area. 


Form 

Area. 

No.  of 
Fluc- 
tuat's 

ISt 

Vis. 

Av. 
Vis. 

Total 
Vis. 

Av. 

Invis. 

Total 
Invis. 

Vis. 
Invis. 

Strip 

.5x2    cm. 

0 

38.5 

38.5 

38.5 

o 

0 

38.5 

Strip 
Square 

•5^5      " 
1.5x1.5  " 

3 
o 

6.2 

'49.0 

12.5 

49.0 

50.0 
49.0 

4.0 

0 

1.2 

o 

51-2 
49  -o 

Strip 
Square 

•  5  x  10    " 

2.2  X  2.2    " 

4 

0 

56.0 

8-5 
56.0 

42.5 
56.0 

0-3 

0 

1.2 

O 

43-7 
56.0 

Strip 
Square 

.5X20      " 
3.IX3.I    " 

7 

i 

67.0 

7.6 
35-8 

60.8 
71.6 

0-35 
1.9 

2.2 
1-9 

63.0 
73-5 

Strip 
Square 

•5  x  40    " 

4.4x4.4   " 

5 

2 

4.5 

66.0 

n.  i 
24-5 

66.6 
73-5 

0-39 
i.i 

1-9 
2.2 

68.5 
75-7 

io6 


FERRKK 
TABLE  XXXVI.     (Observer  W.) 


Form 

Area 

No.  of 

Fluct- 
uat's 

ist 
Vis. 

Av. 
Vis. 

Total 

Vis. 

Av. 
Invis. 

Total 
Invis. 

Vis. 

+ 
Invis. 

Square 

.5*  .5  cm. 

0 

40-5 

40.5 

40-5 

0 

0 

40-5 

Strip 
Square 

•5x5        " 
1-5x1-5 

3 

i 

23-5 
38-5 

18.5 
3i-3 

74.0 
62.  6j 

1.2 

I 

3-6 

i 

77-6 
63.6 

Strip 
Square 

.5x10      " 

2.2X2.2       " 

4 

i 

14.0 
32-6 

13-7 
28.2 

68.5 
56-4 

1.9 

i-5 

5-7 
3 

74.2 
59-4 

Strip 
Square 

.5X20         " 
3-IX3.I 

4 

2 

16.9 
29.4 

14.0 
26.3 

70.0 
78.9 

i-5 

1.2 

6.0 

2.4 

76.0 
81.3 

Strip 
Square 

.5X40         " 
4.4x4.4 

3 
3 

23-0 
26.2 

21.  1 

23 

84-4 
92 

1.8 
1-3 

5-4 
3-9 

89.  3 
95-9 

TABLE  XXXVII.     (Observer  M.) 


Form 

Area 

No.  of 

Fluct- 
uat's 

ist 
Vis. 

Av. 
Vis. 

Total 
Vis. 

Av. 
Invis. 

Total 
Invis. 

Vis. 

+ 
Invis. 

Square 

•  5*  -5  cm. 

0 

48.3 

48-3 

68.3 

o 

o 

68.0 

Strip 
Square 

•5X5        " 
1.5x1.5 

5 
o 

56.0 
68.2 

12.6 

68.2 

75-6 
68.2 

I.O 
0 

5-0 

0 

80.6 
68.2 

Strip 
Square 

.5x10      " 

2.2X2.2       " 

9 

2 

36.6 
55-6 

6.7 
23-3 

67.0 
69.9 

I  .2 

2.O 

10.8 
4.0 

77-8 
73-9 

Strip 
Square 

.5X20         " 
3-IX3.I 

II 

3 

11.4 
82.6 

5-4 
26.0 

64.8 
104.0 

I.I 

2.4 

12.  1 

7-2 

76.9 

III.  2 

Strip 
Square 

.5X40         " 
4.4x4.4 

II 

4 

11.9 
86.6 

19.6 

62.4 
98.0 

1.2 

2.4 

13-2 
9.6 

75-6 
107.6 

Hi.  The  arrangement  of  the  stimulus  with  reference  to  the 
direction  of  greatest  eye-movement  influences  the  frequency  of 
fluctuation  and  the  duration  of  the  after-image.  Again,  the  same 
set  of  stimuli  were  used  as  for  natural  fluctuation,  and  the 
other  conditions  of  the  experiment  were  kept  the  same.  For 
W  the  eye-movement  was  given  in  the  horizontal  plane  with 
both  arrangements  of  the  stimuli.  The  tables  show  that  when 
the  strip  was  arranged  with  its  length  in  the  vertical  plane,  so 
that  the  movement  was  directed  along  its  shorter  dimension, 
the  fluctuations  were  more  frequent  and  the  duration  was 
shorter. 


INTERMITTENCE   OF   MINIMAL  VISUAL  SENSATIONS      1 07 

TABLE  XXXVIII 

W.    Duplication    of  results    by    voluntary  eye-movement.    Showing 

that  the  arrangement  of  the  stimulus  with    reference    to    the 

direction  of  greatest  eye-movement  affects  the  frequency 

of  fluctuation  and  the  duration  of  the  after-image. 


Arrange- 
ment 

Area 

No.  of 
Fluct- 
uat's 

ISt 

Vis. 

Av. 
Vis. 

Total 
Vis. 

Av. 
Invis. 

Total 
Invis. 

Vis. 
Invis. 

Vertical 
Horizontal 

2x5  cm. 
2x5     " 

2 
2 

12.9 

17.4 

n.  i 
16.6 

33-3 
49.8 

o-7 
i.i 

1.4 

2.2 

34-7 
52.0 

Vertical 
Horizontal 

2  X  10    " 
2  X  10    " 

3 
3 

22.8 

33-6 

IO.2 
15-4 

40.8 
61.6 

0.8 

I.O 

2-4 

3-o 

43-2 
64.6 

Vertical 
Horizontal 

2  X  20   " 
2  X  20    " 

4 

4 

25-5 
39-o 

8.9 
13-2 

44-5 
66.0 

0.7 
0.9 

2.8 

3-6 

47-3 
69.6 

Vertical 
Horizontal 

2  X  40    " 
2  X  40    " 

5 
5 

22.4 
38.6 

9.2 
15-0 

55-2 
90.0 

0.8 

1.2 

4.0 
6.0 

59-2 
9.6 

Vertical 
Horizontal 

2  X  50    " 

2  X  50    " 

5 
4 

18.2 
£26.3 

8.7 
14.8 

52.2 

74.0 

I.I 

1-3 

5-5 
5-2 

57-7 
79.2 

The  law  that  eye-movement,  when  directed  along  the  lesser 
dimension  of  the  after-image,  is  more  effective  to  produce 
fluctuation  and  to  shorten  duration  was  given  a  still  more 
thorough  verification  in  the  cases  of  A  and  S.  The  strip  was 

TABLE  XXXIX.    (Observer  A.) 


Arrange- 
ment 
Of  Strip 

Dimensions 
of 
Strip 

Direction 
of 
Movement 

No.  of 
Fluct- 
uat's 

Av. 
Vis. 

Total 
Vis. 

Av. 
Invis. 

Total 
Invis. 

Vis. 
+ 
Invis. 

Horizontal 
« 

.5x10  cm. 
« 

Vertical 
Horizontal 

4 
3 

3-3 
14.8 

16.5 
59-2 

0.8 
0.9 

3-2 
2.7 

19.7 
61.9 

Vertical 
<« 

« 
« 

Horizontal 
Vertical 

5 

i 

5-8 

22.0 

34-8 
44.0 

o-5 
1.4 

2-5 
1.4 

37-3 
45-4 

Horizontal 
« 

.5x20  cm. 
« 

Vertical 
Horizontal 

8 

i 

5-i 

25.0 

45-9 
50.0 

0.6 
0-5 

4.8 
o-5 

50-7 
50-5 

Vertical 
« 

« 
« 

Horizontal 
Vertical 

6 

2 

16.4 

22.4 
49.2 

0.6 
i.i 

3-6 

2.2 

26.0 
5i-4 

Horizontal 
« 

.5x40  cm. 
n 

Vertical 
Horizontal 

3 

i 

13-2 

34-7 

52-8 
69.4 

0.4 
o-3 

1.2 
0-3 

54-o 
69.7 

Vertical 
« 

« 

Horizontal 
Vertical 

6 

4 

4.0 

10.  1 

28.0 
50-5 

0.4 
o-5 

2.4 
2.0 

30.4 
52-5 

io8 


FERREE 


placed  with  its  length  in  the  horizontal  plane,  and  eye-move- 
ment prescribed  first  in  the  vertical  and  then  in  the  horizontal 
plane.  Then  the  strip  was  placed  with  its  length  in  the  verti- 
"cal  plane,  and  movement  prescribed  first  in  the  horizontal  and 
then  in  the  vertical  plane.  It  is  thus  shown  that  the  law  is 
not  dependent  upon  the  plane  in  which  the  strip  is  arranged, 
or  the  direction  of  the  eye-movement,  but  that  the  only  essen- 
tial condition  is  that  the  movement  be  along  the  lesser  dimen- 
sion of  the  after-image. 

XL.     (Observer  S.) 


Arrange- 
ment of 
Strip 

Dimensions 
of  Strip 

Direction 
of  Move- 
ment 

No.  of 
Fluct- 
uat's 

ISt 

Vis. 

Av. 
Vis. 

Total 
Vis. 

Av. 
Invis. 

Total 
Invis. 

Vis. 
Invis. 

Vertical 

.5  x  locm. 

Horizontal 

5 

21 

5-9 

35-4 

2.6 

13.0 

48.4 

" 

.5  x  10   " 

Vertical 

2 

30 

14-3 

42.9 

3-2 

6.4 

49-3 

Horizontal 

.5  x  10   " 
.5x10  " 

Horizontal 

4 
3 

34 
42 

9.0 
13-7 

45-o 
54-8 

2.0 

3-3 

8.0 
9-9 

64.7 

Vertical 

.5x20   " 

ii 

16 

19 

4.1 

69.7 

2.0 

32.0 

101.7 

" 

.5x20   " 

Vertical 

9 

43 

7-9 

79-o 

2-7 

24-3 

103-3 

Horizontal 

.5x20  " 
.5x20  " 

Horizontal 

8 
6 

li 

7-3 
ii-5 

65-7 
80.5 

2-7 

13-6 
16.2 

79-3 
96.7 

Vertical 

.5x40   " 

.< 

8 

45 

8.7 

78.3 

2.6 

20.8 

99.1 

" 

•5^40  " 

Vertical 

5 

56 

14-1 

84.6 

3-i 

15-5 

IOO.I 

Horizontal 

•  5x40   " 

M 

6 

63 

13-3 

93-1 

2.0 

12.0 

105.1 

n 

.5x40   " 

Horizontal 

4 

78 

20.0 

100.  0 

2-3 

9-2 

109.2 

(6)  An  increase  in  the  time  of  stimulation  increases  the  num- 
ber of  fluctuations  of  the  after-image.  Hering  standard  blue, 
10  by  10  cm.,  was  used  as  stimulus  for  A  and  M;  the  same 

TABLE  XLI 

A.    Increase  of  time  of  stimulation  increases  frequency  of  fluctuation 
of  after-image. 


Time  of 
Stimulation 

No.  of 
Fluct- 
uat's 

ISt 

Vis. 

Av. 
Vis. 

Total 
Vis. 

Av. 
Invis. 

Total 
Invis. 

Vis. 
Invis. 

10  sec. 

i 

42.0 

27-5 

55 

5-4 

5-4 

60.4 

40    " 

4 

78.4 

23-8 

119 

3-i 

12.4 

I3I-4 

70    « 

7 

97.6 

20.  2 

162 

2.4 

16.8 

178.8 

90    " 

8 

94-3 

17.4 

157 

2.1 

16.8 

173-8 

INTERMITTENT  OF  MINIMAL  VISUAL  SENSATIONS      109 

blue,  5  by  5  cm.,  was  used  for  W.  The  stimulus  was  placed 
on  a  square  of  engine-gray  cardboard,  and  fixated  at  its  centre. 
The  after-image  was  projected  upon  a  similar  cardboard,  with 
a  fixation  point. 

It  will  be  noticed  that,  with  a  stimulation  of  10  sec.,  the 
after-image  began  fluctuating  8.4  sec.  before  its  final  disap- 
pearance; with  40  sec.,  53  sec.  before;  with  70  sec.,  81.2  sec. 
before;  and  with  90  sec.,  87.5  sec.  before  disappearance.  In- 
crease in  the  time  of  stimulation  results,  then,  in  the  after-image 
beginning  to  fluctuate  at  a  greater  intensity.  If  this  result  is  taken 
in  connection  with  the  proof  of  an  increase  of  eye-movement 
for  the  longer  times  of  stimulation,  it  affords  a  strong  indication 
that  eye-movement  causes  fluctuation.  The  conclusion  is  made 
almost  positive  by  the  fact  that  increase  of  intensity,  without 
increase  of  time  of  stimulation!,  does  not  increase  fluctuation. 

TABLE  XLII.    (Observer  W.) 


Time  of 
Stimulation 

No.  of 

Fluctuat's 

ISt 

Vis. 

Av. 

Vis. 

Total 
Vis. 

Av. 

Invis. 

Total 
Invis. 

Vis. 
+ 
Invis. 

10  sec. 

o 

23-9 

23-9 

23-9 

o 

o 

23-9 

40    " 

2 

13-3 

19-3 

38.7 

1-7 

3-4 

42.1 

70    « 

3 

22.1 

16.8 

64.4 

i-7 

5-i 

69-5 

The  table  shows  that  with  a  stimulation  of  10  sec.  the  after- 
image did  not  fluctuate  at  all;  with  40  sec.  it  began  to  fluctuate 
28.8  sec.,  and  with  70  sec.,  47. 4  sec.  before  final  disappearance. 

TABLE  XLIII.     (Observer  M.) 


Time  of 
Stimulation 

No.  of 
Fluctuat's 

ISt 

Vis. 

Av. 
Vis. 

Total 
Vis. 

Av. 

Invis. 

Total 
Invis. 

Vis. 
Invis. 

10  sec. 

0 

18.0 

18.0 

18.0 

o 

0 

18.0 

40     «•;.- 

4 

42.0 

10.7 

53-5 

3-o 

12.0 

65-5 

100      " 

6 

48.0 

9.8 

68.8 

3-2 

19.0 

87.8 

An  inspection  of  the  table  will  show  that  with  a  stimulation  of 
10  sec.,  the  after-image  did  not  fluctuate  at  all;  with  40  sec., 
it  began  to  fluctuate  23.5  sec.,  and  with  100  sec.  39.8  sec. 
before  it  finally  disappeared. 

That  increase  in  eye-movement  follows  increase  in  the  time  of 
stimulation  was  proved  by  the  second  method  for  the  investi- 


no 


FERREE 


gation  of  eye-movement:  i.  e.,  by  a  direct  record  of  the  move- 
ment of  the  projected  after-image.  The  following  results  were 
obtained. 

TABUS  XLIV 

A.    Eye-movement  with  variation  in  time  of  stimulation.    Showing 

that  increase  in  time  of  stimulation  increases  the  involuntary 

eye-movement  occurring  when  after-image  is  observed. 


Time  of 
Stimulation 

Time 
Observed 

Time 
Mov- 
ing 

Time 
Still 

Time  Moving 
Time'still 

10  sec. 

14-7 

4-95 

9-75 

o-5 

70    " 

54-5 

26.4 

28.1 

0.94 

100      " 

41.0 

23.0 

18.0 

1.28 

130  " 

52-2 

30-4 

22.3 

1.36 

180    " 

46.0 

24.4 

21.65 

1.  12 

TABLE  XLV.     (Observer  W.) 


Time  of 
Stimulation 

Time 
Observed 

Time 
Moving 

Time 
Still 

Time  Moving 
Time'still 

10  sec. 

64-35 

22.75 

41.6 

0-54 

40     " 

85.0 

29.0 

46.0 

0.85 

70    " 

96.5 

48.?5 

48.2 

I.O 

130    " 

102.0 

55-3 

46.7 

1.16 

190    " 

95-o 

59-85 

35-15 

i-7 

TABLE  XL VI.    (Observer  M.) 


Time  of 
Stimulation 

Time 
Observed 

Time 
Mov- 
ing 

Time 
Still 

Time  Moving 
Time'still 

10  sec. 

22.5 

3-25 

19-25 

O.II 

40    " 

61.0 

25-75 

35-25 

0-73 

70    " 

5^-4 

29.65 

22.75 

1.30 

It  was  stated  under  the  heading  'Results:  General'  that  in- 
crease in  the  time  of  stimulation  gave  three  results:  it  increased 


INTERMITTENCE  OF   MINIMAI,   VISUAL  SENSATIONS      III 

the  intensity  of  the  after-image,  the  amount  of  involuntary 
eye-movement,  and  both  the  number  of  fluctuations  and  the 
intensity  at  which  fluctuation  began.  In  order  to  determine 
positively  which  of  the  two  first  is  the  cause  of  the  third,  it 
was  necessary  to  study  the  effect  of  increase  of  intensity  in 
isolation,  z.  e.,  to  increase  intensity  without  increase  of  invol- 
untary eye-movement.  Since  increase  in  involuntary  eye- 
movement  is  found  to  follow  increase  in  time  of  stimulation, 
the  increased  intensity  must  be  secured  without  increase  of 
stimulation  time.  This  was  done  as  follows:  Hering-gray, 
no.  31,  was  used  as  stimulus  background.  Squares  5  by  5  cm. 
of  Hering  grays  15  and  7,  and  of  Hering  white,  were  used  in 
turn  as  stimuli.  The  intensity  of  the  stimulus  in  this  case  is 
measured  by  its  difference  from  the  background.  Thus  the 
intensities,  roughly  at  least,  stood  in  the  relation  16  :  24  :  31. 
A  square  of  Hering  gray  no.  15  was  used  as  the  background 
upon  which  to  project  the  after-images.  This  shade  of  gray 
was  selected  because  it  corresponded  approximately  to  the 
after-effect  of  the  stimulus  background.  Thus  the  projection 
background  was  kept  constant  until  the  after-image,  whose 
fluctuations  were  being  observed,  finally  disappeared.  This 
precaution  may  not  have  been  necessary,  but  it  seemed  well 
to  plan  the  experiment  as  carefully  as  possible.  The  time  of 
stimulation  was  40  sec.  throughout.  All  these  conditions  were 
the  same  for  the  different  stimuli. 

A  typical  set  of  averages  has  been  selected  for  publication. 
It  will  be  observed  that  increase  of  intensity,  without  increase 
in  time  of  stimulation,  does  not  increase  either  the  number  of 
fluctuations,  or  the  intensity  at  which  fluctuation  begins. 
Hence  these  results,  when  obtained  with  increase  in  time  of 
stimulation,  must  be  due  to  increase  of  eye-movement. 

TABI,B  XI/VII 

A.    Results  showing  that  increase    in    intensity   does    not   increase 
fluctuation  of  after-image. 


Stimulus 

No.  of 
Fluct- 
uat's 

ISt 

Vis. 

Av. 
Vis. 

Total 
Vis. 

Ay. 

Invis. 

Total 
Invis. 

Vis. 
Invis. 

Hering  Gray  No.  15 
on      "           "        «    31 

5 

20.3 

7-4 

44.4 

2.7 

13-5 

57-9 

Hering  Gray  No.  7 

on      "           "        "    31 

5 

22.0 

9-3 

55-8 

1.9 

9-5 

65-3 

Hering  White 
on      "        Gray  No.  31 

3 

28.0 

15-6 

62.4 

2-3 

6.9 

69-3 

112  FKRRKE 

(7)  The  observers  most  sensitive  to  the  methods  used  to  disturb 
fixation  show  the  widest  range  of  variability  in  fluctuation  and 
duration.     This   will   be   seen   by  a   comparison   of  the  eye- 
movement  with  the  after-image  tables  for  each  of  the  observers 
and  for  the  various  methods  used.     Dr.  Bair  (B)  and  Miss 
Alden  (A)  were  the  most  sensitive;  the  Misses  Montgomery 
(M)  and  Wright  (Wr)   the  least;  and  Miss  Walter  (W)  was 
of  intermediate  sensitivity. 

(8)  Increase  of  practice  in  fixation  brought  with  it  a  decrease 
in  the  frequency  of  fluctuation  and  an  increase  in  the  duration  of 
the  after-image.     Space  does  not  permit  us  to  show  in  detail 
this  falling  off  in  sensitivity  of  the  different  observers  as  the 
work  progressed.     It  will  be  sufficient  to  say  that  it  was  quite 
marked. 

iii.    How  does  eye-movement  cause  the  fluctuation  and  shorten  the 
duration  of  the  after-image  ? 

a.  It  is  evident  that  neither  Fechner's  nor  Helmholtz' 
theory  is  adequate  to  the  results  given  in  the  preceding  Sec- 
tions. Changes  in  illumination  (Helmholtz)  do  not  account  for 
the  shape  of  the  curve  of  frequency  for  variation  of  area. 
Nor  do  they  explain  the  fluctuation  of  the  after-image  in  parts, 
or  the  effect  produced  upon  fluctuation  by  variations  in  the 
form  and  arrangement  of  the  stimulus.  Fechner's  theory, 
that  eye-movement  arouses  vascular  and  nervous  disturbances 
which  in  turn  react  upon  the  after-image,  is,  first  of  all,  too 
indefinite.  We  are  not  told  how  these  disturbances  work,  and 
no  tangible  evidence  is  adduced  that  eye-movement  produces 
them.  In  the  second  place,  even  if  the  disturbances  are 
granted,  it  is  difficult  to  understand  why  they  take  place  in 
this  and  that  part  of  the  retina  while  the  remainder  is  not 
affected  (fluctuation  in  parts);  why  they  are  effective  in  the 
case  of  certain  areas,  and  not  at  all  in  that  of  others  (effect  of 
variation  of  area  on  fluctuation);  and,  still  more,  why  the 
form  of  the  stimulus,  and  its  arrangement  with  regard  to  the 
direction  of  greatest  range  and  frequency  of  eye-movement, 
etc.,  affect  the  fluctuation  and  duration  of  the  after-image  as 
powerfully  as  they  are  found  to  do. 

Fick  and  Giirber  follow  a  different  course.  They  study  the 
relief  of  adaptation  not,  like  Helmholtz  and  Fechner,  from  the 
side  of  the  negative  after-image,  but  from  the  positive  side. 
They  show  in  various  ways  that  the  color  or  brightness  of  a 
stimulus  to  which  the  eye  has  been  adapted  is  restored  by  eye- 
movement.  They  contend  that  adaptation  is  a  phenomenon  of 
fatigue,  and  that  eye-movement  relieves  it,  chiefly,  by  facili- 
tating the  removal  of  the  fatigue  products  from  the  retina;  less 
importantly,  by  increasing  the  delivery  of  new  material  to  the 


INTERMITTENCE   OF   MINIMAL  VISUAL  SENSATIONS      113 

fatigued  end-organs.  This  hypothesis,  though  perhaps  the 
most  promising  of  all  the  eye-movement  theories,  is  at  the  same 
time  scarcely  less  speculative  than  the  others.  The  passage  of 
lymph  to  and  from  the  retinal  elements  is  a  necessary  postulate 
of  metabolism ;  but  Fick  and  Giirber  give  no  direct  or  positive 
proof  that  eye-movement  facilitates  the  exchange;  nor  has  the 
proof  been  brought  by  any  subsequent  investigator. 

Vascular  disturbances  iii  the  retina  are  alleged  as  indirect  evidence. 
The  following  authorities  may  be  cited  upon  this  point.  On  the  nega- 
tive side,  we  find  A.  Coccius  (Ueber  die  Anwendung  des  Augenspiegels, 
etc.,  1853,  20),  who  was  the  first  to  investigate  the  matter,  asserting 
that  the  disturbances  are  not  present  in  the  case  of  quick,  short  eye- 
movements.  O.  Becker  (Archiv  f.  Ophthalmol.,  XVIII,  I,  1872,  242) 
contends  that  eye-movement  exerts  no  especial  influence,  since  he 
finds  fluctuations  in  the  caliber  of  the  retinal  vessels  when  the  eye- 
muscles  are  paralyzed  by  atropine.  On  the  positive  side,  A.  v.  Graefe 
(Archiv  f.  Opthalmol.,  I,  2,  1855,  387)  establishes  the  general  principle 
that  eye-movement  causes  an  increase  of  pressure  in  the  vitreous 
humor ;  hence  every  change  of  fixation  is  followed  by  an  increase  of 
vascular  pulsation  in  the  retina.  Michell  (Lehrbuch  der  Augenheil- 
kunde,  i.  Aufl.,  547)  observed  that  eye-movement  causes  a  paling  of 
the  retinal  capillaries.  W.  Dobrowolsky  (Centralblatt  f.  d.  medic. 
Wissensch.,  1870,  20  and  21),  working  on  a  dog,  observed  frequent 
changes  in  the  calibre  of  the  retinal  capillaries.  These  changes  dis- 
appeared, however,  when  motor  paralysis  was  produced  by  curare; 
the  capillaries  also  became  paler  when  the  eye-muscles  were  electri- 
cally stimulated.  Fick  and  Giirber  themselves,  working  both  on  the 
human  eye  and  on  the  eyes  of  dogs,  were  able  in  some  cases  to  observe 
the  effect  of  eye-movement  on  the  calibre  of  the  retinal  vessels.  They 
found,  e.  g.,  in  the  case  of  the  human  eye,  that  when  the  eyes  are  held 
steadily  upon  some  distant  object,  vascular  changes  are  not  noticeable, 
but  that  noticeable  changes  occur  when  the  eyes  are  moved.  They 
believe  that  these  changes  are  not  normal  pulsations  due  to  the  heart's 
action,  but  are  directly  caused  by  the  eye-movements.  The  natural 
pulse,  they  say,  is  not  observable  for  various  reasons  :  thus,  it  may 
possibly  be  obscured  by  the  rapidity  of  the  heart's  action.  This  sup- 
position seems  to  receive  confirmation  from  the  results  of  experiments 
on  dogs.  When  the  dogs  were  put  under  the  influence  of  chloral  or 
morphine,  and  all  the  eye-muscles  severed,  a  rhythmical  change  in 
the  calibre  of  the  retinal  vessels  was  plainly  noticeable.  Since  eye- 
movement  could  not  operate  to  produce  this  rhythm,  they  regard  it  as 
that  of  the  natural  retinal  pulse,  now  rendered  observable  through 
the  slowing  of  the  heart's  action  by  the  drugs  used.  On  the  whole, 
however,  and  having  regard  both  to  their  own  work  and  to  that  of 
others,  Fick  and  Giirber  did  not  consider  the  evidence  that  eye- 
movement  influences  retinal  circulation  to  be  entirely  satisfactory. 

Moreover,  if  we  look  at  the  problem  from  the  side  of  the 
negative  after-image,  the  hypothesis  can  apparently  be  of  ser- 
vice only  in  explaining  the  effect  of  eye-movement  upon  the 
total  duration  of  the  after-image.  Facilitation  of  the  removal 
of  fatigue  products  does  not  account  for  fluctuation;  for  if  the 
fatigue  material  is  carried  away  so  completely  as  to  cause  the 
disappearance  of  the  after-image,  there  is  no  satisfactory  or 
plausible  reason  why  it  should  accumulate  again,  and  cause  the 

JOURNAL — 8 


114  FERREB 

after-image  to  reappear,  when  the  eye  has  in  the  meantime  un- 
dergone no  additional  stimulation.1  Since,  then,  the  hypothe- 
sis cannot  explain  a  simple  case  of  fluctuation,  it  manifestly 
cannot  account  for  the  variations  in  the  phenomenon  discussed 
in  the  foregoing  pages; 2  and,  failing  in  this  regard,  it  mani- 
festly has  not  taken  into  consideration  all  the  factors  that 
operate  to  relieve  adaptation. 

b.  Much  to  our  surprise,  the  solution  of  the  problem  came 
with  the  observation  of  a  phenomenon  which,  for  want  of 
a  better  term,  will  be  called  the  'streaming  phenomenon.'  We 
turn  to  its  consideration  with  the  reluctance  that  a  writer  must 
feel  in  pointing  out  a  new  phenomenon  in  a  field  so  old  and  so 
minutely  canvassed  as  that  of  vision,  but  nevertheless  with  a 
full  sense  of  responsibility.  The  phenomenon  was  first  ob- 
served in  1905,  and  since  that  time  has  been  carefully  investi- 
gated by  the  writer  with  the  aid  of  nine  observers  :  seven  stu- 
dents of  psychology  and  two  laymen.  All  were  sceptical  at  the 
outset;  but  later,  independently  of  one  anothers'  and  of  the 
writer's  observations,  were  able  to  describe  the  phenomenon  in 
detail,  and  to  sketch  the  more  prominent  of  the  multitude  of 
stream  patterns. 

i.  A  brief  description  is  difficult.  When  one  sits  with 
lightly  closed  lids,  which  must  be  kept  from  quivering,  before 
a  bright  diffuse  light,  such  as  that  of  a  partly  clouded  sky, 

1  Fick  and  Giirber  do  not  treat  their  problem   from  the   side  of  the 
negative  after-image,  nor  are  they  primarily  concerned  with  this  as- 
pect of   adaptation  (especially  with  the  fluctuation  of  the  negative 
after-image);  at  the  same  time,    they  offer  an   explanation  of  this 
fluctuation.    The  explanation,  however,  does  not  account  for  the  vari- 
ous cases  and  types  of  fluctuation;  and  its  basis  is  both  hypothetical 
and,  in  terms  of  the  results  of  our  own  observers,  contrary  to  fact.    It 
runs  as  follows  :  "Augenbewegungen  und  Accommodation  quetschen 
die   Netzhaut  gleichsam   aus,   das   negative  Nachbild  verschwindet. 
Aber  nicht  aus  der  ganzen  Netzhaut  werden  die  Stoffwechselproducte 
entfernt,  sondern  nur  aus  der  empfindlichsten  Schichte;  darum  taucht 
das  negative  Nachbild  in  demselben  Masse  wieder  auf,  in  dem  sich  die 
Stoffwechselproducte  wieder  iiber  die  empfindlichste  Netzhautschi- 
chte  verbreiten"  (Archiv  f.  Ophthalmol.,  XXXVI,  300).     Observation 
shows  rather  that  instead  of  the  waste  material  being  forced  from  the 
after-image  area  by  eye-movement,  and  returning  to  it  when  the  intra- 
ocular pressure  is   relieved,  just  the   reverse   movement  of  material 
takes  place.     That  is,  eye-movement  causes  a  wash  of  material  from 
some  part  of  the  surrounding  retina  over  the  after-image   area.     As 
long  as  this   streaming  material   is  passing  over  the  area,  the  after- 
image cannot   be   seen.     When  it  has  passed  beyond  it,  the  image  re- 
appears. 

2  It  is  not  clear,  e.  g.,  why  the  waste  material  forced  from  the  after- 
image area  does  not  return,  to   cause  the  reappearance  of  large  and 
small  after-images,  as  it  does  so  readily  in  the  case  of  after-images  of 
medium  area.     Similar  difficulties,  too  obvious  to  need  separate  men- 
tion, are  encountered  in  the  other  cases  discussed  above. 


INTBRMITTBNCE  OF  MINIMAL  VISUAL  SENSATIONS      115 

and  looks  deep  into  the  field  of  vision  thus  presented,  beyond 
the  background  as  usually  observed,  one  sees  about  the  point 
of  regard,  after  the  field  of  vision  has  steadied,  slowly  moving 
swirls.1  These  swirls  have  the  appearance  of  streams  of 
granules  moving  in  broad  curves  now  this  way,  now  that, 
seemingly  without  order  unless  a  noticeable  eye-movement 
occurs,  or  is  made  voluntarily,  when  the  direction  of  streaming 
changes  to  that  of  the  eye-movement.2  The  change  of  direc- 
tion is  always  on  a  curve,  the  abruptness  of  which  depends 
upon  the  vigor  of  the  movements,  much  as  would  happen  if 
motions  in  different  directions  and  of  different  magnitudes  were 
compounded  at  intervals  upon  a  fluid  of  considerable  inertia. 
The  phenomenon  is  extremely  varied.  Sometimes  the  central 
portion  of  the  field  of  vision  resembles  the  surface  of  a  liquid 
about  to  boil,  channeled  this  way  and  that  by  convection  cur- 

1  The  manner  in  which  the  lids  are  held  is  of  extreme  importance. 
They  must  not  be  closed  so   tightly  that   pressure   is   exerted  on  the 
eyeballs,  and  on  the  other  hand,  they  must  not  admit  light.     It  is 
difficult  at  first  to  find   just  the   right  background  and  the  proper 
illumination.     Quivering  of  the  lids  is  fatal  to  the  observation.    The 
writer's  only  failure  to  secure  a  successful  observation  was  on  the  part 
of  an  observer  who  could  not  keep   the  lids  from   trembling.     Such 
painstaking  precautions  are  necessary,  however,  only  until  the  ob- 
server has  once  seen  the  phenomenon.     Afterwards  there  is  no  diffi- 
culty.    In   fact,  like   the   entoptic  and   circulation   phenomena,  the 
streaming  may  even  become  troublesome  by  its  insistence.     The  plane 
of   fixation   is   a    matter   of   peculiar  consequence.     One  must  look 
through  and  beyond  the  shifting,  changing,  indefinite  haze  that  occu- 
pies the  visual  foreground  of  the  closed  eyes.     No  better  direction 
can  be  given  than  that  the  observer  try  to  resolve  this  haze,  to  find 
out  what  lies  in  and  behind  it.     He  must  gaze   intently  and   pene- 
tratingly.    Since  the  smoothest  part  of  the  closed  lid  lies  above  the 
centre  of  the  eye,  it  is  of  advantage  to  look  slightly  upward,  instead 
of  directly  forward.     The   steady  field   of  vision   should  reveal   the 
streaming,  but  voluntary  eye-movement,  by  increasing  its  activity, 
frequently  facilitates  the  observation.     If  one  moves  the  eye  sharply, 
and  intently  watches  the  field  of  vision  in  the  trail  of  the  movement, 
one  sees  (sometimes  immediately  following  and  sometimes  lagging 
behind;  a  stream  which  takes  the  general  direction  of  the  movement. 
By  way  of  final   caution,  we    cannot  emphasize  too  thoroughly  the 
need  of  persistence  and  patience  in  observing.     An  unpractised  ob- 
server should  not  expect  to  see  the  phenomenon  in  less  than  2-5  min- 
utes after  closing  his  eyes.     The  field  of  vision  must  clear  and  settle, 
and  the  observer  must  grow  accustomed  to  the  unusual  conditions  of 
observation. 

2  The  real  movement  of  the  streaming  is  in  the  opposite  direction  to 
that  of  the  retina.     The  apparent  motion  of  the  eye  is  the  movement 
of  its  anterior  portion.     This   is  opposite  to  the  movement  of  the 
retina;  hence  the  streaming  material  seems  to  pass  across  the  field  of 
vision    in    the   direction   in  which  the  eye  is  moving.     It  might,  of 
course,  be  expected  that  a  mobile  material  on  the  retina  would  move 
under  the  influence  of  eye-movement  as  the  streaming  material  is 
thus  found  to  do. 


1 1 6  FKRREE 

rents  moving  at  varying  rates  of  speed.  Now  and  again  a 
heavy  stream  will  sweep  across  this  channeled  surface  from  one 
direction  or  another,  taking  up  the  minor  swirls  as  sharply 
curving  tributaries,  and  so  on,  through  manifold  changes. 
Various  patterns  can  be  picked  out,  and  a  particular  swirl  may 
be  traced  in  its  deviations  for  a  time;  but,  as  a  whole,  the 
phenomenon  cannot  be  adequately  described. 

After  practice  on  the  closed  lids,  the  observers  became  able 
to  trace  the  streaming  on  any  dull  or  rough  surface  with  the 
eyes  open.  It  may  also  be  observed  under  the  conditions  of 
observation  of  the  entoptic  and  circulation  phenomena;  but 
just  as  one  must  look  beyond  the  false  scotomata  to  see  the 
moving  corpuscles  and  interspaces,  so  must  one  look  beyond 
them  to  see  the  streaming.  So  competent  did  certain  observers 
become,  with  the  eyes  open,  that  records  were  made  in  the 
experiments  with  minimal  stimuli  of  the  time  that  heavy 
streaming  lasted  during  the  phases  of  invisibility.1 

2.  In  casting  about  for  a  physiological  explanation  of  the 
streaming  phenomenon,  we  have  been  led  to  believe  that  it  is 
caused  by  a  streaming  over  the  retina  of  some  material  which 
is  capable  of  directly  affecting  the  processes  that  condition 
visual  sensation.  First,  on  the  negative  side,  we  find  that  it 
cannot  be  a  circulation,  entoptic,  or  tear-film  phenomenon,  or 
any  of  the  shadow  phenomena ;  for  it  is  seen  in  the  dark  as  well 
as  in  the  light.  In  fact  the  best  way  to  observe  it,  with  the 
eyes  open,  is  in  the  dark  room  or  in  a  carefully  muffled  black- 
ness cylinder.  Secondly,  on  the  positive  side,  we  have  three 
facts  to  consider.  The  streaming  occurs,  as  we  have  just  men- 
tioned, in  the  dark  as  well  as  in  the  light.  It  is  seen  in  the 
dark  as  a  streaming  and  swirling  of  the  intermingled  blackness 
and  luminous  haze  that  compose  the  visual  field.  Here  it  must 
directly  excite  the  black- white  process.  Again,  the  streams 
carry  with  them  the  quality  of  the  background  from  which 
they  proceed.  This  fact  may  be  demonstrated  as  follows.  Get 
upon  the  retina  a  large  square  blue  after-image,  having 
through  its  centre  a  vertical  strip  of  yellow.  Projected  upon 
the  field  of  vision  afforded  by  the  closed  lids  in  daylight,  this 
image  will  be  seen  as  a  strip  of  reddish  yellow  on  a  back- 
ground of  purple.  A  heavy  stream,  in  passing  across  the  strip, 
will  sweep  the  purple  with  it  across  the  yellow,  and  will  de- 
posit traces  of  the  yellow  in  irregular  patches  on  the  further 
side.  In  other  cases,  where  a  heavy  swirling  takes  place  over 
the  strip  area,  the  yellow  will  break  up  irregularly,  traces  and 

1The  phenomenon  comes  out  with  remarkable  clearness  with  open 
eyes,  in  the  blackness  of  the  dark  room.  The  field  of  slightly  lumin- 
ous haze  that  there  confronts  one  for  some  minutes,  at  a  certain  dis- 
tance, streams  and  swirls  with  convincing  distinctness. 


INTERMITTENCE   OF   MINIMAL  VISUAL  SENSATIONS      Iiy 

patches  of  its  color  being  borne  out  in  different  directions,  and 
the  background  swept  in.  In  yet  other  cases,  the  swirl  will 
form  outside  the  image,  and  sweep  across  it,  much  as  a  swirl 
of  snow  is  carried  before  the  wind.  In  all  of  these  variations 
of  stream-type,  however,  it  is  the  unfailing  rule  that  the  qual- 
ity of  the  background  is  carried  by  the  stream  from  point  to 
point  in  its  course.  Lastly,  the  streaming  has  a  characteristic 
effect  upon  the  after-image.  Gentle  streaming  dims  the  after- 
image, apparently  in  proportion  to  its  vigor,  provided  that  it 
comes  from  an  area  of  different  visual  quality,  but  heavy 
streaming  blots  it  out  absolutely.  This  occurrence,  once  seen, 
can  never  be  doubted.  The  observation  is  positive. 

3.  The  effect  of  streaming  is  the  same,  whether  the  eyes  are 
open  or  closed.  It  is  true  that  the  stream  is  not  so  clearly  seen 
to  sweep  over  the  after-image  when  this  is  projected  on  a  back- 
ground with  the  eyes  open,  as  when  it  is  projected  on  the  field 
of  the  closed  lids.  Nevertheless,  the  instant  that  the  image 
disappears,  streaming  can  always  be  plainly  seen  over  the  area 
which  it  occupied.  If  the  background  is  a  rough,  dull  surface, 
the  stream  can  even  be  seen  to  form  and  pass  across  the  image. 
Most  of  our  observers  readily  noticed  this  phenomenon,  and 
some  reported  it  before  they  had  discovered  streaming  in  the 
field  of  the  closed  lids,  so  that  it  formed  their  first  observation 
of  streaming.1 

We  have,  further,  indirect  evidence  of  the  identity  of  fluc- 
tuation under  the  two  conditions  of  projection,  in  the  striking 

1  A  word  of  explanation  here  may  prevent  misunderstanding.  What 
was  observed,  with  the  eyes  open,  was  that  the  background  seemed  to 
sweep  over  this  or  that  part  of  the  after-image  area,  and  that  as  the 
stream  advanced  the  color  disappeared.  The  stream  seemed,  decid- 
edly, to  sweep  the  color  out,  or  at  least  to  be  causally  connected  with 
its  disappearance.  The  phenomenon  is,  naturally,  less  striking  than 
it  is  when  the  after-image  is  seen  on  the  field  of  the  closed  lids.  For 
one  thing,  the  image  is  now  projected  on  a  field  some  distance  away; 
the  streams  are  thus  magnified,  and  accordingly  rendered  vague  and 
diffuse,  less  distinct  in  form  and  outline.  For  another  thing,  the 
brightness  of  daylight  illumination  tends  to  obscure  the  phenomenon. 
And  other  factors  in  the  result  could  probably  be  mentioned. 

Many  of  the  brief  disappearances  of  visual  stimuli,  especially  those 
of  positive  stimuli  of  considerable  intensity,  are  seen  to  be  caused  in 
the  same  way  by  streaming.  The  stream  sweeps  over  them  and  blots 
them  out ;  as  it  passes  off,  they  reappear  from  behind,  slightly  farther 
back.  The  latter  phenomenon  is  especially  noticeable  in  the  case  of 
the  shadows  cast  by  a  false  scotoma  on  the  retina. 

Similar  observations  may  be  made  in  the  course  of  adaptation  ex- 
periments. L/ay,  e.  g.,  a  fairly  dark  disc  of  Hering  gray  on  a  back- 
ground of  Hering  gray  several  shades  darker.  While  the  disc  is 
leveling  down  to  the  background,  this  or  that  portion  of  it  will  be 
repeatedly  swept  out  by  streams,  moving  across  it  in  a  curve.  Many 
of  the  writer's  laboratory  students  at  different  colleges  have  reported 
this  phenomenon. 


Il8  FERREE 

correspondence  which  obtains  between  the  types  of  disappear- 
ance when  the  image  is  observed,  as  is  ordinarily  done,  on  a 
cardboard  or  other  background,  and  when  it  is  traced  on  the 
field  of  the  closed  lids  in  conjunction  with  streaming.  Parallel 
series  of  experiments  were  carried  out  with  after-images  in  the 
form  of  strips,  squares,  and  crosses,  large  and  small,  projected 
both  upon  a  background  of  cardboard  and  upon  the  field  of  the 
closed  lids.  The  result  was  that,  if  a  sufficient  number  of 
cases  is  considered,  every  type  of  disappearance  found  in  the 
one  series  can  be  found  also  in  the  other.  The  following 
paired  observations,  selected  at  random,  will  illustrate  this 
correspondence. 

Stimulus :  Milton  Bradley  standard  yellow,  42  by  4  cm.  After- 
image observed  on  background  of  engine-gray  cardboard.  Time  of 
stimulation  30  sec.  Distance  of  O  i  meter. 

After-image  dimmed  over  its  whole  area.  Revived.  Bottom  went 
out.  Reappeared.  Whole  image  went,  disappearing  first  along  right 
edge  in  form  of  a  curve,  convex  inwardly,  then  spreading.  Reap- 
peared, beginning  along  right  edge.  Top  disappeared.  Reappeared 
almost  immediately.  Top  and  bottom  went  out  almost  simulta- 
neously, slowly  followed  by  centre.  (These  disappearances  rarely  if  ever 
occurred  over  the  whole  area  at  once.  They  began  in  a  certain  part, 
and  then  extended;  one  could  see  the  area  in  process  of  being  swept 
over.  The  parts  disappearing  were  always  bounded  by  curved  lines.) 
Bottom  came  back  first.  Upper  part  disappeared,  central  and  lower 
part  dimmed,  but  did  not  disappear.  Reappeared.  Centre  went  out; 
then  bottom.  Quickly  reappeared.  Whole  after-image  disappeared, 
beginning  in  lower  right-hand  corner  and  spreading  towards  the  top. 
Faintly  reappeared.  Vanished. 

Stimulus:  Milton-Bradley  standard  yellow,  42  by  4  cm.  After-image 
observed  on  the  field  of  the  closed  lids.  Time  of  stimulation,  30  sec. 
Distance  of  O  i  meter. 

Gentle  streaming  aud  dimming  over  the  whole  area  of  the  after- 
image. Upper  half  and  centre  swept  out  by  a  swirl  moving  counter 
clockwise,  and  carried  across  from  the  left.  Reappeared.  Whole  after- 
image blotted  out  by  two  streams  moving  from  left  and  right,  joining, 
and  moving  upwards  for  the  whole  length  of  the  after-image.  Cleared 
over  whole  area,  beginniug  at  bottom.  Lower  part  swept  out  by  a 
stream  moving  obliquely  down  and  to  left.  Almost  immediately  an- 
other swept  up  and  to  right,  carrying  out  centre  and  upper  half. 
Reappeared.  Swirl  counter-clockwise  cut  off  top.  Reappeared. 
Stream  moving  on  a  very  gradual  curve  to  right  from  above  cut  out 
all  but  right  edge  of  lower  half.  Finally  this  became  involved. 
Cleared.  Heavy  stream  moved  across  centre  from  right,  blotting 
it  out;  divided  and  swept  back  on  itself  obliquely  towards  top  and 
bottom, carrying  out  whole  after-image.  General  commotion.  Dimmed. 
Unable  to  follow  changes  farther. 

Stimulus:  Milton-Bradley  yellow,  10  by  10  cm.  After-image  ob- 
served on  engine-gray  cardboard.  Time  of  stimulation,  30  sec.  Dis- 
tance of  O  i  meter. 

Gradually  dimmed.  A  curved  segment  was  blotted  out  of  left  side. 
Reappeared.  Disappeared,  beginning  with  top.  Reappeared,  begin- 
ning with  upper  right-hand  corner.  Disappeared,  beginning  with 
top  and  upper  left  corner.  Left  side  came  back  first,  then  whole  im- 
age. Disappeared,  beginning  at  top.  Lower  left  corner  went  next. 


INTERMITTENCE   OF   MINIMAL  VISUAL  SENSATIONS      1 19 

Lower  right  corner  was  slow  to  go.  Reappeared,  beginning  with 
lower  left  corner.  Disappeared  again  almost  immediately.  Reap- 
peared, beginning  at  bottom.  Quickly  disappeared.  Reappeared 
faintly;  then  vanished. 

Stimulus:  Milton-Bradley  yellow,  10  by  10  cm.  After-image  ob- 
served on  field  of  closed  lids.  Time  of  stimulation,  30  sec.  Distance 
of  O  i  meter. 

Gentle  general  streaming  dimmed  after-image.  Lower  right- 
hand  corner  cut  off  by  streams,  moving  on  short  curves.  Cleared. 
Stream  from  below  moving  from  right  to  left  swept  out  lower 
right  corner  and  bottom.  Cleared.  Stream  swept  from  right  up- 
per corner  diagonally  to  lower  left;  immediately  turning  back  and 
describing  a  sinuous  path,  swept  out  all  but  upper  left  and  lower 
right-hand  corners.  They,  too,  were  soon  involved.  Cleared,  begin- 
ning at  lower  right  and  upper  left  corners.  Two  streams  coming 
from  below,  left  and  right,  joined  and  passed  upwards  across  after- 
image, sweeping  it  out.  They  turned  and  apparently  wound  back 
and  forth  over  after-image  several  times,  causing  a  long  disappear- 
ance. Lower  left  corner  cleared  first.  Light  stream  swept  across 
from  left  to  right  and  almost  blotted  out  image.  It  turned  broadly 
upwards,  then  sharply  downwards  as  a  heavy  stream,  blotting  out  im- 
age completely.  Cleared,  beginning  at  upper  left  corner.  General 
agitation  over  after-image  (convection-current  effect).  Colors  of  back- 
ground and  after-image  mingled  in  the  general  swirling.  After-image 
disappeared.  Cleared.  Stream  started  at  bottom,  bent  towards  right, 
turned  on  itself  towards  left,  again  upwards  and  towards  right,  and 
then  downwards  in  broad  S-shaped  curve.  After-image  was  blotted  out. 
Faintly  reappeared.  Almost  immediately  was  involved  in  a  general 
swirling  and  vanished. 

The  next  two  observations  serve,  further,  to  demonstrate  that 
when  a  stream  sweeps  across  an  after-image  area,  causing  the 
after-image  to  disappear,  it  carries  with  it  the  visual  quality, 
color  and  brightness,  of  the  area  from  which  it  proceeds. 

Stimulus:  a  strip  of  Milton  Bradley  standard  yellow  paper,  42  by  4 
cm.  on  a  sheet  of  Milton-Bradley  standard  blue  paper,  52  by  59  cm.; 
giving  as  after-image  on  the  field  of  the  closed  lids,  a  reddish  blue 
strip  on  a  reddish  yellow  background.  Time  of  stimulation  30  sec. 
Distance  of  O  i  meter. 

First  the  yellow  background  could  be  seen  streaming  gently  over 
the  after-image,  gradually  dimming  it.  Swirl  of  complicated  curves 
blotted  out  whole  after-image.  Cleared  slowly,  and  after-image  could 
be  seen  here  and  there  at  the  less  dense  places.  Gradually  cleared  all 
over.  Circular  swirl  cut  out  centre  and  about  two-thirds  of  upper 
half.  Cleared.  Another  swirl  cut  out  small  area  at  centre  and  moved 
diagonally  upwards  and  towards  right.  Centre  cleared.  Streams, 
sweeping  from  right  to  left,  joined  at  bottom  of  after-image,  and 
traversed  its  entire  length,  returning  upon  themselves  on  curves 
having  the  shape  of  an  ellipse.  After-image  cleared  beginning  at 
bottom.  Lower  part  of  after-image  cut  off  by  stream  sweeping  across 
to  right,  which  turned  and  went  obliquely  towards  left  and  upwards, 
cutting  out  centre.  Cleared  at  bottom  first.  General  swirling.  At 
some  places  after-image  was  carried  out  into  background,  and  back- 
ground carried  in;  so  that  after-image  presented  irregular  outline. 
Soon  swirling  became  more  violent  and  after-image  and  background 
intermingled.  After-image  became  indistinguishable.  Did  not  re- 
appear. (In  every  disappearance  the  stream  could  be  seen  to  carry 
the  yellow  of  the  background  over  the  strip.  This  was  especially 


120  FERRBE 

noticeable  when  the  disappearance  was  progressive,  from  part  to 
part.  When,  e.g.,  a  swirl,  expanding,  involved  successively  more 
and  more  of  the  image,  the  front  of  swirling  yellow  could  be  seen  to 
encroach  upon  the  blue  at  each  revolution.) 

Stimulus:  a  strip  of  Milton-Bradley  standard  yellow  paper,  42  by  4 
cm.  on  a  sheet  of  Milton-Bradley  standard  blue  paper,  52  by  59  cm. ; 
giving  as  after-image  on  a  sheet  of  engine-gray  cardboard  a  strip  of 
blue  on  a  background  of  yellow.  Time  of  stimulation  30  sec.  Dis- 
tance of  O  i  meter. 

Dimmed  all  over.  Went  out  at  centre,  then  at  top  and  bottom. 
Reappeared.  Bottom  swept  out;  then  whole  image.  Reappeared. 
Section  in  middle  of  upper  half  went  out,  followed  almost  immediately 
by  section  in  middle  of  lower  half.  Both  reappeared.  Whole  after- 
image blotted  out.  Reappeared,  first  at  bottom.  Top  and  bottom 
went  out,  followed  by  centre.  Centre  reappeared  first.  Long,  slender, 
crescent-shaped  section  disappeared  from  right  hand  upper,  then  from 
left  hand  lower  portion.  Reappeared  faintly,  beginning  with  right 
hand  upper  corner;  then  whole  image  disappeared.  Reappeared. 
Lower  part  disappeared.  Reappeared  momentarily;  then  whole  image 
vanished.  Reappeared  faintly,  and  vanished.  (At  each  disappear- 
ance, the  after-image  area  occupied  by  the  blue  strip  took  on  the  yellow 
of  the  surrounding  background,  instead  of  preserving  the  gray  of  the 
cardboard.  In  many  cases  it  could  be  seen  to  do  this  progressively; 
*.  e.,  the  yellow  would  begin  at  a  corner,  an  edge,  etc.,  and  spread  in 
the  direction  in  which  the  disappearance  of  the  strip  was  taking  place. 
The  phenomenon  was  very  clear,  e.  g.y  when  a  corner,  bottom,  or  what 
not  went  out,  and  the  disappearance  spread,  finally  involving  the 
whole  image.  This  type  of  disappearance,  when  observed  on  the 
closed  lids,  generally  showed  a  swirling  stream  which  spread  centrifu- 
gally  until  the  whole  after-image  area  was  swept  over.) 

4.  The  effect  of  streaming  on  the  flight  of  colors  is  three- 
fold. First,  a  gentle  streaming  may  merely  dim  the  color. 
Secondly,  more  intensive  streaming  advances  the  color  changes 
one  or  more  stages.  When  the  change  is  advanced  only  one 
stage,  the  image  sometimes  does,  and  sometimes  does  not  re- 
turn to  the  preceding  stage,  when  the  stream  has  passed  over. 
When  the  change  is  advanced  two  or  more  stages,  the  image 
apparently  always  returns  to  the  initial  stage,  or  to  the  initial 
stage  but  one,  when  the  stream  has  cleared  away.  Thirdly, 
when  the  streams  are  strong  and  heavy,  the  image  is  blotted 
out  completely,  returning  to  the  initial  stage  when  the  stream 
has  passed  on  either  abruptly,  or  quickly  through  the  series  of 
color  changes, — most  frequently  in  their  inverse  order,  but 
sometimes  in  irregular  order.  These  effects  cannot  be  due  to 
a  shadow  cast  by  the  stream  upon  the  retina;  for  shadows,  like 
any  reduction  of  the  illumination  of  the  retina,  push  the  color 
change  in  the  inverse  direction.  Hence  we  have  here  addi- 
tional evidence  that  the  locus  of  the  physiological  condition  of 
streaming  is  not  anterior  to  the  retina. 

The  following  is  a  description  of  the  effect  of  streaming  on 
the  flight  of  colors.1  An  account  of  the  phenomenon  as  it 

1  All  the   phenomena   described   in   connection  with  streaming  arc 


INTERMITTENCE  OF   MINIMAL  VISUAL  SENSATIONS      121 

appears  under  still  better  conditions  has  been  given  above 
(after-image  from  the  sun's  disc). 

The  stimulus  was  afforded  by  a  triangular  opening,  i  by  2  ft. , 
in  a  curtain  near  the  ceiling  of  a  closely  curtained  room.  0 
was  seated  about  10  ft.  from  this  opening,  and  looked  up 
through  it  directly  at  the  brightly  illuminated  sky.  The  sash 
of  the  window  was  lowered.  In  line  with  the  centre  of  the 
opening  was  a  patch  of  bright,  fleecy  clouds.  Thus  the  stimu- 
lus consisted  of  a  brilliant  blue  triangle  with  a  bright  white 
patch  at  its  centre.  When  stared  at  for  a  long  time,  e.  g.,  for 
5  min.,  the  triangle  underwent  the  following  qualitative 
changes.  The  white  patch  at  its  centre  became  gradually  dim- 
mer. Finally,  it  became  completely  overcast  with  the  sur- 
rounding blue.  This  was  concomitant  with  intensive  streaming 
of  the  swirling  type.  It  cleared  somewhat;  then  the  whole 
triangle  changed  to  a  deep,  saturated  pink.  After  remaining 
in  this  stage  for  a  short  time,  it  changed  again  to  blue,  and 
finally  once  more  to  pink.  Here  the  observation  ceased. 
When  the  after-image  was  to  be  traced,  O  fixated  the  centre 
of  the  opening  for  about  20  sec. ,  and  then  covered  the  closed 
eyes  with  a  black  cloth.  The  observation  was  thus  made  in  a 
well- darkened  field  of  vision.  The  report  is  as  follows. 

After-image  developed  as  light  faintly  reddish  blue.  Fluctuated 
several  times  between  this  and  yellowish  green.  (Thus  far,  as  it  hap- 
pened, the  streaming  was  not  intensive.)  Finally  settled  into  yellow- 
ish green.  Heavy  streams  frequently  swept  across,  carrying  it  com- 
pletely out.  On  reappearing,  it  came  directly  back  to  yellowish  green 
once.  The  remaining  times  it  reappeared  first  as  deep  red,  then 
quickly  changed  into  yellowish  green.  The  yellowish  green  area 
grew  smaller  as  fluctuation  went  on,  leaving  a  growing  border  of  deep 
red.  Next  fluctuation  began  from  yellowish  green  to  deep  red,  as  a 
result  of  less  intensive  streaming.  The  yellowish  green  area  was 
swept  off,  as  if  it  were  an  upper  layer,  leaving  the  purplish  red  under- 
neath and  further  back.  One  could  see  the  yellowish  green  streaming 
raggedly  beyond  the  after-image.  This  was  very  plain  at  times. 
Once  a  narrow  stream  was  observed  to  cut  a  channel  through  the 
yellowish  green  near  the  toe  of  triangle,  exposing  the  deep  red  appar- 
ently beneath.  Disappearances  also  took  place  at  this  stage  as  result  of 
heavy  streaming.  After-image  finally  settled  down  into  deep  red. 
This  layer  seemed  to  be  noticeably  farther  back  in  the  field  of  vision 
than  the  other  color  layers.  Disappearances  were  quite  frequent,  and 
were  plainly  the  result  of  streaming.  This  stage  lasted  relatively  a 
long  time.  Next  faint  stages  respectively  of  dark  blue  and  dull  dark 
yellow  were  noticeable.  Fluctuations  were  frequent  and  disappear- 
ances long.  There  was  the  usual  connection  between  fluctuation  and 
streaming. 

noticeably  plainer  when  the  observation  is  made  in  the  higher  alti- 
tudes. Whether  this  result  is  due  solely  to  the  condition  of  illumina- 
tion there  found,  which  facilitates  observation  (especially  when  made 
in  daylight  on  the  field  of  the  closed  lids),  and  conceivably  induces  a 
more  or  less  special  retinal  state,  or  whether  indirect  physiological 
influences  are  at  work,  the  writer  has  no  means  of  deciding. 


122 

The  following  is  a  variation  of  the  streaming  phenomenon, 
not  observed  in  connection  with  after-images.  It  is  given  as 
possibly  throwing  some  light  on  the  physiological  condition  of 
streaming  from  a  slightly  different  angle.  The  observation 
was  made  out  of  doors  in  Colorado,  early  in  October,  near  the 
middle  of  a  cloudless  day.  The  light  stimulation  was  very 
intensive. 

Sometimes  when  the  eyes,  having  been  exposed  to  the  bright  diffuse 
light,  were  closed,  and  the  field  of  vision  had  settled,  one  saw  scat- 
tered over  it  here  and  there,  and  fairly  close  together,  islands  or 
patches  slightly  darker  than  the  background,  presenting  a  porous 
appearance,  due  to  a  peculiar  mottling  of  lighter  and  darker  gray. 
These  patches  were  not  after-images.  They  were,  however,  readily 
seen  to  be  carried  along  and  broken  up  by  the  streams,  the  parts 
taking  the  velocity  of  the  stream  current.  Streams  could  be  seen  to 
cut  channels  through  groups  of  the  patches.  The  patches  themselves 
seem  to  be  conditioned  by  some  mobile  material,  which  shares  in  the 
general  streaming. 

Another  phenomenon,  which  seems  to  indicate  that  there  is 
even  a  mass  mobility  of  the  material  which  conditions  visual 
sensation,  is  described  as  follows.  The  observation  was  made 
under  the  same  conditions  as  the  last. 

When  one  has  two  or  more  after-images  of  the  sun's  disc  close  to- 
gether, they  will  often  be  seen  soon  to  merge  into  one.  Sometimes  a 
channel  cuts  across  from  the  one  to  the  other,  and  the  two  gradually 
draw  together  into  a  more  or  less  circular  form.  Again,  the  one  will 
be  seen  to  move  bodily  towards  the  other,  finally  uniting  with  it. 

5.  That  there  is  a  dependence  of  streaming  upon  eye-move- 
ment cannot  be  doubted.  This  dependence  is  shown  in  two 
ways.  First,  when  an  eye-movement  is  noticeable,  or  is  made 
voluntarily,  a  heavier  stream  is  started  in  the  apparent  tangle 
of  swirling  in  the  direction  of  the  eye-movement.  Secondly, 
whenever  involuntary  eye-movement  is  increased,  as  by  previous 
long  fixation,  by  a  faulty  centering  of  the  after-image  upon  the 
retina,  etc.,  greater  commotion  is  noticeable  over  the  streaming 
area.  That  there  is  a  movement  of  this  material,  independent 
of  the  effect  of  eye-movement,  is  also  probable;  but  the  heavy 
streams  that  intermittently  blot  out  the  after-image  are  doubt- 
less determined  by  eye-movement.1 

1What  the  nature  of  the  streaming  material  is  we  shall  not  attempt 
to  decide,  further  than  to  point  out  that  the  results  show  it  to  be  visu- 
ally active.  Metabolism  requires  that  there  be  a  diffusion  of  lymph 
over  the  retina.  We  might,  then,  identify  the  streaming  material  with 
this  metabolic  substance,  making  it  the  vehicle  of  both  catabolic  and 
anabolic  processes.  The  anabolic  material  is  conceivably  in  part  dis- 
integrated visual  substance  which  retains  for  a  time  its  power  to  con- 
dition visual  sensation,  as  is  shown  by  the  streams  bearing  with  them 
the  visual  quality  of  the  region  from  which  they  come.  Thus  by 
weakening  the  after-image  through  hastening  metabolic  change,  and 
by  setting  up  strongly  the  sensation  of  the  region  from  which  the 


INTERMITTENCE  OF   MINIMAI,  VISUAL  SENSATIONS      123 

c.  When  once  we  have  established  the  connection  between 
fluctuation  and  eye-movement  on  the  one  hand,  and  eye-move- 
ment and  streaming  on  the  other,  explanation  goes  compara- 
tively smoothly.  The  results  obtained  by  varying  the  steadi- 
ness of  fixation  and  by  increasing  the  time  of  stimulation 
present  no  especial  problem.  There  is  in  these  cases  merely 
an  increase  or  decrease  of  eye-movement,  and  a  corresponding 
increase  or  decrease  in  the  streaming  activity,  with  a  resultant 
increase  or  decrease  in  fluctuation.  With  the  remaining  meth- 
ods, however,  the  situation  is  different.  They  will  be  consid- 
ered in  turn. 

(i)  We  have  found  that  fluctuation  occurs  only  within  a 
limited  region  of  after-image  areas.  Probably  the  most  difficult 
problem  that  fluctuation  sets  to  theory  is  this  effect  of  variation 
of  area.  Before  it,  the  oscillatory  theory  seems  to  break  down 
absolutely. 

Considered  with  regard  to  area,  after-images  naturally  fall 
into  four  classes  :  small  images  which  fluctuate  little  or  not  at 
all  (for  our  observers,  images  with  an  area  of  1.5  by  1.5  cm. 
and  less) ;  larger  images  which  fluctuate  over  their  whole  area  ; 
still  larger  images  which  fluctuate  in  parts ;  and  quite  large 
images  which  do  not  fluctuate  at  all.  Now  this  grouping  is 
just  what  we  might  expect  from  the  nature  of  the  streaming 
phenomenon.  It  derives  directly  from  the  following  facts : 
that  streaming  does  not  cause  disappearance  unless  the  stream 
comes  from  a  part  of  the  retina  undergoing  different  stimulation; 
that  the  streams  have  a  more  or  less  definite  form,  i.  £.,  that 
they  sweep  across  this  or  that  part  of  the  after-image,  preserving 
pretty  clearly  defined  borders;  that  vigorous  streaming  appar- 
ently occurs  only  over  a  somewhat  limited  region  about  the 
point  of  regard;  and  that  the  centre  of  the  field  of  vision  is 
always  in  a  state  of  more  or  less  violent  swirling.  Here  the 
streams  are  narrow  and  swift-moving; l  as  we  pass  towards 

streams  come,  heavy  streaming  may  temporarily  obscure  the  after- 
image. It  is,  however,  useless  to  work  out  in  detail  what,  with  our 
present  kaowledge  of  visual  processes,  can  at  best  be  but  a  mere  con- 
jecture. We  desire  to  lay  stress  upon  nothing  except  the  observed 
facts. 

1The  direction  of  streaming  is,  as  we  have  seen,  determined  or  mod- 
ified by  eye-movement.  The  lines  of  direction  described  by  all  the 
points  on  the  moving  retina,  for  the  various  directions  in  which  the 
eye  moves,  crowd  together  in  its  central  portion;  hence  all  the 
streams,  since  their  directions  are  determined  by  the  moving  retina, 
converge  towards  its  centre.  This  accounts  for  the  narrowing  of  the 
streams,  and  the  consequent  more  rapid  motion  of  the  streaming  ma- 
terial. More  or  less  continuous  commotion  at  the  centre  must  also 
result  from  these  conditions;  and  the  twisted,  swirling,  tangled  pat- 
terns are  produced  by  the  many-times  compounded  motions.  The 
centre  of  the  retina  is  thus  least  liable  to  adaptation  and  after-image 


124  FERREE 

the  periphery,  the  swirling  becomes  more  diffuse,  and  the 
streams  are  broader,  and  more  widely  separated  both  in  space 
and  time. 

For  convenience  of  discussion,  therefore,  the  retinal  field  may 
be  divided  into  four  zones,  each  one  of  which  usually,  but  not 
always,  contains  different  stages  of  the  same  stream.  The 
streams  form  near  the  periphery  of  the  retina,  and  tend  to  move 
towards  its  centre.  In  the  first  or  central  zone,  streaming  is 
practically  continuous.  Here  the  streams  are  narrow,  swift, 
and  often  crowded  together.  The  second  zone  is  made  up  in 
part  of  cross-sections  of  the  streams  found  in  the  first,  and  in 
part  of  streams  whose  directions  have  been  changed  before  they 
reached  the  first  zone.  Here  the  streams  are  broader  and  less 
swift  and  vigorous.  The  streaming  at  any  given  point  is  dis- 
continuous; a  given  segment  of  the  zone  is  streamed  over  at 
irregular  intervals.  In  the  third  zone  are  found,  in  general, 
the  source  and  the  upper  courses  of  the  streams  passing  across 
the  first  and  second  zones.  The  streaming  here  is  still  more 
discontinuous,  and  the  streams  are  still  broader  and  more 
sluggish.  The  fourth  zone  lies  beyond  the  area  of  observable 
streaming. 

The  infrequency  or  entire  absence  of  fluctuation  of  the  small 
images  of  the  first  group  is  explained  by  the  fact  that  they  lie 
wholly  within  the  first  zone,  the  region  of  most  active  commo- 
tion. Hence,  when  they  have  once  reached  the  dimness  at 
which  disappearance  begins,  there  is  not  sufficient  lull  in  the 
streaming  for  reappearance  to  occur.  The  records  show  less 
eye- movement  for  the  images  of  the  first  than  for  those  of  the 
second  group.  Here  we  come  upon  an  exception  to  the  general 
law  of  the  effect  of  eye-movement  upon  fluctuation;  for  this  re- 
duction of  eye-movement  favors  rather  than  decreases  the 
fluctuation  of  these  small  images.  It  is,  however,  readily  in- 
telligible that  a  more  continuous  eye-movement,  by  causing 
more  continuous  streaming,  would  give  even  less  opportunity 
for  reappearance. 

The  images  of  the  second  group  extend  beyond  the  limits  of 
the  central  zone;  and  the  continuous  streaming  of  this  region 
does  not  cause  them  to  disappear,  since  to  do  so  the  streams 
must  come  from  a  region  of  different  stimulation.  They  lie, 
however,  within  the  second  zone,  where  the  streaming  is  still 
vigorous  though  not  continuous.  A  broad  heavy  stream 
sweeps  in  from  the  outside,  and  may  either  blot  out  the  whole 
image  at  once  (this  depends  upon  the  relative  sizes  of  stream 

effects.  This  conclusion  tallies  with  well-known  facts  of  vision. 
The  centre  of  the  retina  is  the  region  of  clearest  vision;  adaptation 
there  takes  place  less  quickly, and  the  correlated  after-images  develop 
more  slowly. 


INTERMITTENT   OF   MINIMAL  VISUAL  SENSATIONS      125 

and  image),  or  may  at  first  efface  only  a  part.  Then,  as  the 
stream  compounds  with  other  streams,  the  whole  image  will 
become  involved.  When  the  commotion  subsides,  the  image 
reappears,  remaining  until  blotted  out  by  another  stream  or 
stream-system.  Since  we  are  now  in  a  region  of  only  occasional 
streaming,  increase  of  eye-movement  must  increase  the  frequency 
of  fluctuation,  by  increasing  the  number  of  streams  that  sweep 
across  the  image.  There  is,  however,  sufficient  intermission 
in  the  streaming  for  reappearance  to  take  place.  Continuous 
voluntary  eye-movement  would  probably  produce  continuous 
streaming  over  this  region;  but,  under  the  conditions  of  ordi- 
nary fixation,  we  have  a  fluctuation  which  is  proportional  to 
the  frequency  and  range  of  eye-movement. 

The  images  of  the  third  group  are  still  included  in  the  region 
of  streaming.  They  lie  within  the  borders  of  the  third  zone, 
but  are  too  large  to  have  their  whole  area  involved  at  one  time 
by  a  single  stream  or  stream-system.  So  we  have  the  phe- 
nomenon of  fluctuation  in  parts.  As  in  the  previous  case,  in- 
crease of  eye-movement  increases  streaming,  and  accordingly 
increases  frequency  of  fluctuation. 

The  images  of  the  fourth  group  cover  the  whole  area  of 
effective  streaming.  Their  borders  lie  in  the  fourth  zone;  and 
consequently  fluctuation  does  not  occur.  The  streams  that 
twist  about  over  the  surfaces  of  these  images  do  not  come  from 
a  region  of  different  stimulation,  consequently  do  not  blot  them 
out.1 

(2)  We  found  that  the  form  of  the  stimulus  affects  the  fre- 
quency of  fluctuation  and  the  duration  of  the  after-image. 
The  stimuli  used  were  strips  and  squares  of  equivalent  areas. 
We  now  have  to  explain,  first,  the  shape  of  the  curve  of  fre- 
quency for  the  strip-images.  An  increased  length  of  strip, 
while  it  produces  practically  the  same  effect  on  eye-movement 
as  increased  area  of  square,  does  not  give  the  same  shape  to 
the  curve.  WTith  the  squares  the  curve,  after  reaching  its 
maximal  height,  bends  down  rather  sharply  to  the  abscissae; 

1  The  explanation  in  this  and  the  following  sections  may  seem 
somewhat  complicated;  but  the  facts  are  themselves  complicated. 
The  results  of  observation  are  recorded  as  they  were  obtained,  in  no 
wise  modified  to  suit  the  needs  of  theory.  The  phenomena  of  stream- 
ing and  the  phenomena  of  fluctuation  were  investigated  independently 
and  at  different  times.  The  details  of  streaming,  its  patterns,  zones, 
etc.,  were  worked  out,  and  in  part  verified,  a  full  year  before  the  re- 
sults on  fluctuation  given  in  the  foregoing  tables  were  obtained. 
After  those  results  had  been  obtained,  however,  for  variations  in  size, 
form,  arrangement,  etc.,  of  stimulus,  the  present  investigations  were 
begun,  with  projection  of  the  images  on  the  field  of  the  closed  lids,  in 
order  to  determine  the  relation  of  the  various  types  of  fluctuation  to 
the  various  types  of  streaming.  Thus  our  theory  is,  in  reality,  a  de- 
scription of  what  actually  takes  place  in  observation. 


126  FERREE 

with  the  strips,  it  dips  down  very  little.  The  explanation  is 
that  the  squares,  as  they  grow  larger,  come  to  include  the 
whole  of  the  noticeable  streaming  area,  while  the  strips  do  not. 
The  strips  can,  accordingly,  always  be  swept  across  by  streams 
coming  from  a  region  of  different  stimulation.  They  can  be 
blotted  out,  while  the  squares  cannot.  The  shape  of  the  curve 
for  the  strips  is  very  like  that  for  the  squares  until  the  maximal 
height  is  reached;  up  to  this  point  streaming  affects  both  strips 
and  squares  alike. 

We  have  to  explain,  secondly,  the  fluctuation  of  strips  and 
the  partial  absence  of  fluctuation  of  squares  of  equivalent  area. 
The  explanation  lies  in  the  difference  of  the  retinal  zones. 
The  squares  fall  within  the  first  and  the  innermost  part  of 
the  second  zone.  The  strips,  in  proportion  as  they  are  in- 
cluded within  the  first,  second  and  third  zones,  show  the  phe- 
nomena of  fluctuation  characteristic  of  these  regions. 

(3)  We  found  that  the  arrangement  of  the  stimulus,  with 
reference  to  the  direction  of  greatest  eye-movement,  affects  the 
frequency  of  fluctuation  and  the  duration  of  the  after-image. 
Narrow  strips  of  varying  length  undergo  more  frequent  fluc- 
tuations and  have  a  shorter  duration  when  the  direction  of 
greatest  range  and  frequency  of  eye-movement  is  across  jhe 
strip,  than  when  the  inverse  arrangement  obtains.  The  reason 
is  clear:  the  streams  must  be  more  effective  to  produce  disap- 
pearance when  they  sweep  across  the  narrow  after-image,  than 
when  they  traverse  its  length.  Suppose,  e.  g. ,  that  a  narrow 
strip  is  placed  with  its  length  first  in  the  vertical  and  then  in 
the  horizontal  plane.  Let  the  eye-movement,  in  both  cases, 
take  place  in  the  horizontal  plane.  The  vertical  strip  can  be 
more  effectively  swept  by  streams  coming  from  a  region  of 
different  stimulation  than  can  the  horizontal  strip.  Accord- 
ingly we  find  greater  frequency  of  fluctuation  and  a  shorter 
duration  of  the  image  in  the  former  case  than  in  the  latter. 
Conversely,  when  the  movement  is  in  the  vertical  plane,  and 
the  strip  is  arranged  first  vertically  and  then  horizontally,  the 
opposite  effect  should  be  produced.  The  tables  show  that 
this  is  the  case. 

In  the  experiments  on  natural  fluctuations,  the  greater  range 
and  frequency  of  eye-movement,  for  all  observers,  were  in  the 
horizontal  plane.  Hence  greater  frequency  of  fluctuation  and 
shorter  duration  should  have  been  observed  when  the  strip  was 
arranged  with  its  length  in  the  vertical  plane.  The  tables 
show  that  this  expectation  was  realized.1 

1  The  character  of  the  disappearance  is  somewhat  different,  accord- 
ing as  it  is  due  to  voluntary  eye-movement  or  to  natural  fluctuation. 
In  the  former  event  it  is  more  abrupt,  and  more  nearly  covers  the 
whole  length  of  the  image.  In  natural  fluctuation,  the  image  usually 


INTERMITTENCE   OF   MINIMAL  VISUAL  SENSATIONS      127 

(4)  Having  thus  ascertained  the  part  played  by  streaming 
in  the  determination  of  the  duration  and  fluctuation  of  the 
after-image,  we  can  understand  how  it  is  that  results  ob- 
tained when  the  fluctuations  were  produced  by  involuntary 
eye-movement,  varying  in  amount  from  method  to  method, 
could  be  duplicated  by  results  obtained  when  the  fluctuations 
were  produced  by  voluntary  eye-movement,  constant  from 
method  to  method.  There  are  two  possible  reasons.  First, 
there  was  present  in  both  cases  a  variable  amount  of  involun- 
tary eye-movement;  and  secondly  there  was  in  both  cases  the 
same  distribution  of  the  zones  of  streaming. 

There  can  be  no  doubt  that  voluntary  eye-movement,  while 
it  lessened,  did  not  entirely  prevent  involuntary  movement. 
In  the  variation  of  this  latter,  from  method  to  method,  we 
might  find  a  basis  for  the  variation  in  results  obtained,  and 
therefore  an  explanation  of  the  duplication.  On  the  other 
hand,  there  is  strong  evidence  that  the  concomitant  involun- 
tary eye-movement  was  not  the  direct  cause  of  fluctuation. 
The  disappearances  always  followed  directly  upon  the  volun- 
tary movements,  which  must,  therefore,  be  regarded  as  the 
immediate  cause  of  fluctuation.  The  involuntary  movements 
could  have  functioned  only  indirectly,  by  way  of  weakening 
the  after-image,  under  certain  experimental  conditions,  and 
thus  rendering  it  more  liable  to  obliteration  by  the  voluntary 
movements.  Obviously,  then,  the  distribution  of  the  zones  of 
streaming  is  the  more  important  factor. 

It  is  not  difficult  to  see  how  these  two  factors  co-operated  to 
produce  our  results.  If  there  were  no  variation  of  involuntary 
eye-movement,  from  method  to  method,  strict  duplication  should 
result  when  the  fluctuations  are  produced  by  voluntary  eye- 
movement.  If  it  were  not  for  the  identical  distribution  of  the 
zones  of  streaming,  duplication  could  not  result  at  all.  A  con- 
sideration of  the  two  factors  together  enables  us  to  explain  the 
results  obtained. 

(5)  The  peculiarities  of  fluctuation  in  indirect  vision  are 
readily  explained  as  a  result  of  the  distribution  of  the  zones  of 

goes  out  in  successive  parts,  quickly,  until  it  has  disappeared.  The 
difference  reflects  the  nature  of  the  eye-movement.  The  voluntary 
movement  is  a  single  sweep,  out  and  back,  of  considerable  strength 
and  range.  A  broad  current  of  the  streaming  material  is  thus  carried 
across  the  image  in  the  direction  of  the  eye-movement,  and  blots  it 
out  at  once.  The  involuntary  movements  occurring  in  the  case  of 
natural  fluctuation  are  irregular  in  direction,  range  and  frequency, 
and  usually  come  in  groups.  They  therefore  start  a  number  of  streams 
in  different  directions,  and  usually  in  quick  succession.  One  stream 
is  often  seen  to  sweep  across  this  part,  another  across  that;  until 
finally  the  whole  image  becomes  involved  before  any  part  has  had 
time  to  clear. 


128  FERREE 

streaming.  When  a  small  after-image  is  observed,  first  in  the 
central  part  of  the  field,  and  then  successively  farther  and  far- 
ther out  from  the  centre,  there  is  first  an  increase  in  fluctua- 
tion, then  a  decrease,  and  finally  an  entire  cessation.  Now  we 
have  seen  that  an  image  in  the  central  zone  of  streaming,  once 
it  has  disappeared,  is  kept  from  reappearing  by  the  continuous 
commotion  there  present.  As  it  passes  from  the  central  zone 
outward,  into  the  region  of  occasional  streaming,  fluctuation 
must  increase  up  to  a  certain  point  (probably  the  limits  of  the 
second  zone),  and  thereafter  decrease,  ceasing  entirely  when  the 
image  passes  beyond  the  range  of  noticeable  streaming. 

(6)  Fechner,1  Helmholtz,2  and  others  maintain  that  blinking 
and  movement  of  the  head,  as  well  as  movement  of  the  eyes, 
cause  the  after-image  to  disappear.  Both  of  these  movements, 
however,  result  in  eye-movement,  and  hence  may  be  supposed 
to  be  only  indirectly  causes  of  disappearance. 

With  regard  to  blinking,  O.  Weiss  says:8  "Beim  L,idschluss 
zeigt  sich  eine  Bewegung  des  Bulbus  erst  nach  oben  innen,  dann 
nach  oben  aussen."  This  is  called  Bell's  phenomenon.4  The 
movement  can  easily  be  felt  when  one  presses  the  finger  with 
moderate  firmness  on  the  lids,  above  and  to  the  temporal  side 
of  each  bulb,  and  blinks  vigorously.  Von  Michel5  thinks  that 
this  movement-complex  is  controlled  by  the  cortex,  while 
Nagel6  believes  it  to  be  a  reflex,  due  to  the  pressure  of  the 
edges  of  the  lids  upon  the  cornea.  However  this  may  be,  there 
is  distinct  eye-movement,  in  at  least  two  directions,  with  every 
closing  and  opening  of  the  lids;  that  is  to  say,  there  is  ample 
ground  for  considering  eye-movement  to  be  the  more  imme- 
diate cause  of  the  disappearance  of  the  after-image. 

Again,  even  if  the  eye  were  stationary  in  its  socket,  move- 
ment of  the  head  would  affect  the  streaming  phenomenon  very 
much  as  movement  of  the  eyes  does.  The  streaming  material 
would  traverse  the  retina  in  the  opposite  direction  to  that  of 
the  movement.7  But  the  eye  is  not  thus  stationary;  movement 
of  the  head  results  either  in  a  movement  of  the  eye  in  the  oppo- 
site direction,  or  in  this  together  with  a  readjustment  of  the 
eye  in  accordance  with  the  changed  position  of  the  head.  And 
there  is,  further,  a  rotation  of  the  eye  about  its  horizontal  axis, 
which,  according  to  Bonders,  opposes  the  rotation  of  the  head 

1Ann.  d.  Phys.  u.  Chem.,  L,  1840,  221. 

2  Phys.  Optik,  510. 

8Nagel's  Handbuch  d.  Physiol.  des  Menschen,  III,  1905,  471. 

4Philos.  Transact,  of  the  Royal  Soc.,  1823,  166,  289. 

5Beitr.  z.  Physiol.,  Festschr.  f.  Pick,  1899,  159. 

6Archiv  f.  Augenheilk.,  XUII,  199. 

7  This  would  be  in  the  same  direction  as  the  movement  of  the  field 
of  vision,  the  converse  of  what  happens  with  eye-movement.  The  effect 
oa  the  after-image,  however,  would  be  essentially  the  same. 


INTER MITTENCE   OF   MINIMAL  VISUAL  SENSATIONS      1 29 

and  is  of  equal  amount  in  both  eyes.  In  the  case  of  a  sudden 
inclination  of  the  head,  Mulder1  found  a  momentary  torsion  of 
20°.  His  conclusions  as  regards  permanent  torsion  bear  out 
those  of  Skrebitzky.2  Nagel8  found  that  movements  of  torsion 
occur  if  the  head  or  the  head  and  body  together  are  passively 
moved. 

In  fine,  then,  the  effect  upon  the  after-image  of  blinking 
and  of  movements  of  the  head  presents  no  especial  problem  to 
theory;  in  both  cases  definite  and  measureable  eye-movements 
take  place.  Eye-movement,  as  determining  or  modifying  the 
streaming  phenomenon,  explains  fluctuation  under  these  con- 
ditions as  readily  as  it  explains  the  fluctuations  which  occur 
under  the  conditions  of  normal  fixation. 

III.     CONCLUSIONS  AND  RESTATEMENT  OF  THESIS 

The  conclusions  to  be  drawn  from  the  foregoing  experiments, 
with  regard  to  the  fluctuation  and  duration  of  the  negative 
after-image,  are  as  follows,  (i)  The  fluctuation  of  the  nega- 
tive after-image  represents  a  real  intermission  of  sensation.  It 
is  not  an  artifact,  due  to  observation  under  the  conditions  of 
light  adaptation,  for  it  occurs  as  readily  in  a  darkened  as  in  a 
light  field  of  vision.  (2)  Fluctuation  is  not  grounded  in  the 
nature  of  the  after-image  process.  It  is  caused  chiefly  by  in- 
voluntary eye-movement.  (3)  Eye-movement  causes  the  fluc- 
tuation and  decreases  the  duration  of  the  negative  after-image 
by  conditioning, or  modifying  the  streaming  over  the  retina  of 
some  material  capable  of  affecting  the  visual  processes. 

Enlarged  and  restated  in  the  light  of  these  conclusions  our 
thesis  is  this,  (i)  The  intermitteuce  of  minimal  visual  sensa- 
tion is  a  phenomenon  of  adaptation.  (2)  Adaptation  is  ren- 
dered intermittent  chiefly  through  the  influence  of  eye-move- 
ment. (3)  Eye-movement  interferes  with  adaptation  in  three 
ways,  (a)  It  decreases  the  total  time  of  stimulation.  The 
more  eye-movement  there  is,  the  less  intensive  will  be  the  im- 
pression made  upon  the  retina,  (b)  It  affords  time  for  the 
after-image  to  die  away,  or  (in  terms  of  adaptation)  it  gives 
opportunity  for  restoration,  proportional  to  the  length  of  time 
during  which  the  stimulated  area  is  relieved.  And  (c)  more 
immediately,  it  determines  or  influences  the  washing  or  stream- 
ing over  the  retina  of  some  material  capable  of  directly  affect- 
ing the  visual  processes.  Further  evidence  for  this  thesis  will 
be  adduced  in  later  papers. 

1  Archiv  f.  Oplhalmol.,  XXI,  i,  1875,  68. 

2  Archiv  f.  Opthalmol.,  XVII,  i,  1871,  107. 
8  Archiv  f.  Opthalmol.,  XVII,  i,  1871,  237. 


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